Don't beat yourself up or consider yourself foolish. In recognizing your partner as controlling and manipulative, you must also recognize this: Though they can at first be charming, controlling and manipulative people are the unfortunate product of a high, incisive intellect and low self esteem. They are intelligent, usually charismatic people who, at first blush, seem to be confident, charming and together. It's no wonder you found him or her attractive.
Get to the point and don't try to cushion the blow or beat around the bush. Your first instinct is to do it in person (not always advisable, but the honorable first choice) and try to hurt your partner as little as possible, but this may only result in prolonging his/her agony - and yours. Come right out and declare your decision frankly, without hostility or cruelty. S/he will likely be stunned and/or shocked, and may question, attempt to bargain, cry, or become enraged - all are possible reactions. Be prepared for anything.
Depending on your situation, your leaving may be best explained in a note. Be clear about your decision, and then leave at once. Example: "I'm so sorry, but this is not working for me any more, so I'm ending our relationship here. I will always care about you and wish you well, but it's over." Do not say, "I Will Always Love You, XXOO Marty," or "I'll be at my mom's" or "If you need anything let me know." Even if you really do feel this way, this can become the tiny bit of hope s/he needs to continue the obsession with winning you back.
If you feel you must do this face to face, be brief and as dispassionate as possible. (And it's wise to have your things packed and in your car already so that you just need to walk out.) Example: "I just wanted to say this in person. I'm leaving; our relationship has not worked out for me. I wish you well, but I can't continue this," then walk out. Don't look back, despite the fact that s/he is freaking out, shrieking that you can't go, hanging from your pant leg, throwing various objects at you, threatening suicide, and generally having a complete meltdown.
The less personal you can bring yourself to be, the better. It seems cold when your inclination is not to want to hurt your former love, but in reality, the less emotional you are, the less you will escalate the pain. Believe it. Your ex wants to control you and everything you do, and the more s/he realizes that s/he is no longer able to control you, the more intense and hysterical s/he is likely to become. It's an effort to engage your feelings of guilt for hurting him/her, compassion for his or her pain, etc. as they will take no responsibility for the break down of the relationship. S/he will want to get you to respond, as any polite or compassionate person wants to, but once you show any sort of mercy or positive response to this, s/he knows his/her rant has worked, and leaving becomes more difficult for you.
Be decisive and don't fall for promises to change. Once you have identified your relationship as toxic to your individuality and future, you must take decisive steps. Wishy-washy, weak attempts to leave will be steamrolled, and you will be overrun by the will of your partner. Talking things over with your partner will not be likely to help: remember the crucial identifier - this is a controlling manipulator. As soon as you start making noises about being unhappy with controlling behaviors and preparing him or her for the fact you are thinking of ending the relationship, s/he will gladly give in to your desires - just long enough to keep you attached. Stringing you along with little bits of what you need or want makes you believe that s/he has finally heard you, understands your needs, and is willing to change. The problem is that s/he is probably not really capable of changing at this point (as evidenced by no change, despite many so-called efforts, over and over again). As soon as you settle back into the relationship, s/he knows you're back on the hook and the bad behaviors resume. It's all just been a kind of ploy to keep you around, continuing a vicious cycle and allowing him or her to re-establish control.
Leave at once. Having made your decision, waste no time. Notice, this is not the first time the exhortation to leave is made. That's because it's so hard to leave - particularly if you decided a face-to-face farewell was necessary. Please believe that your attempts to leave on good terms will most likely not pan out. The truth is, these efforts will only make it less likely that you will ever be able to have even the most casual of contact with your ex without it turning into a terrible, embarrassing scene. Your caring response instills hope that control can be re-established, and so feeds the obsession with getting you back. - so much so that your partner may abandon all dignity and beg, cry, bargain, scream, etc. If you leave before your ex has completely humiliated him/herself, it really will end better. No matter how hard it is, turn your back on him/her, ignore the begging, sobbing, threatening and yelling, and put some steel in your back. Walk out the door. Shut it behind you.
Stay away. Don't accept phone calls, answer emails, IMs or text messages from him/her. Doing so will only create hope. It's likely to end in an unholy debacle, and things will be worse than ever - you won't just have an angry, upset ex - you'll likely end up with a shrieking harpy freaking out and screeching for your blood. Remember again: this is a controlling, manipulative person who will say anything to win, and that is all this contact will be about. Once you have broken away, stay away. Having dinner alone with him/her, "just to talk" or "for the kids' sake" will destroy your resolve, and will also give your controlling ex the power s/he seeks again. Cut it clean, and let it go.
Avoid mutual friends who are still in contact with your ex for some time after the breakup. The last thing you need is the passing, even inadvertently, of fuel into the fire in the weeks and months after the end of the affair. If you can't avoid contact with these friends, keep your remarks to them carefully neutral, and don't share details of the breakup, your feelings, or your insights on your ex with them - you can almost be assured these remarks will find their way back to your ex, and that will not be a good thing.
Remain detached. In order to reassert control, your ex will look for signs that you are receptive to crying, begging, threats of self-harm, etc. If you simply do not react, you will give no fuel to your ex's belief that s/he can win you back, and it will be truly over much sooner. S/he will cry, rage, rant, become hysterical if you allow him/her to. Being compassionate and trying to comfort or spare your ex further pain will only make it more difficult to break away. Every minute that you stay, talk, commiserate, apologize, or otherwise play along is a win for your ex, because s/he knows you feel helpless to leave him/her in such an awful state. Key word: helpless. Other key word: YOU. Generally think of your ex as a sleeping dragon. The more time s/he stays asleep (e.g., thinks about things other than you), the more likely s/he will stay asleep.
Admit your weakness. Many times, though your partner is controlling and/or manipulative (which is wrong), that partner is exploiting your own weaknesses (which enables the controlling/manipulative behaviors). Though both of you are in the wrong, if you are to avoid the same problems in the future, you will need to address your own insecurities about abandonment, loneliness, and/or your tendencies to wish to "rescue" or by virtue of your love alone, "repair" whatever damage you perceive in others. However, no matter who is most responsible, this situation needs to end. Work out your issues on your own, after separating from this relationship.
Don't delete text messages from your ex, but don't respond to them, either. When you respond, it's a minor win, and continues to feed the notion that a bigger win is in the offing. However, should your ex become stalker-ish, these text messages can provide valuable evidence to the police if you want to get a restraining order. Consider buying a digital recorder, and saving them to a CD-ROM, that you keep in a safe place for if and when the time is necessary.
Cutting off all contact seems cruel, but it's a case of "cruel to be kind." No response = no point in continuing this. Any response = keep trying. The quicker and cleaner your message is received, the sooner s/he will move on to someone else, and you will be free of a potentially explosive situation. The more little tidbits of contact you drop back into your ex's court, the more frustrating it will become for your ex when you continue to resist further efforts. The more frustrated s/he becomes, the more his/her anger and rage become inflamed. No matter how difficult it is, no matter what tender feelings you still have, do not express any of that to your ex. It won't help. It will just make the breakup more difficult for both of you.
Get your support network back. Go to the friends and family you will inevitably have been disconnected from by your controller, fall on your sword, and ask them to take you back. Without trash-talking (or letting others do it, either), you can say, "The bottom line is, you were right, the relationship was toxic, and once I realized it, I got out. I appreciate you taking the risk you did by sharing your misgivings about it with me."
Let's say you have a mutual friend, Jim. As you're driving away, you call Jim and say, "Jim, I just broke up with Laurel. She was pretty upset when I left and you'd do me a big favor by just calling and checking on her, make sure she's okay (don't tell her I asked you to) but I gotta go." Betcha dollars to donuts that Jim will tell you later that she was fine by the time he got there, and didn't seem too upset. You will probably be hearing from s/he that s/he is devastated and can't function and then hear feedback they s/he is out for dinner and continuing on with life but wants you to believe otherwise. The quicker you get it over with, the quicker she'll get over it - as soon as the control issue has been decided (i.e., you've left and you don't dance to her or his tune anymore), there's no need to continue the fight, so the moment you leave, it's over. There may be some residual angst to deal with, but the worst will be past.
Controlling and manipulative people are often produced by external factors that you have no control over. You cannot hope to change or rescue such a person, as much as you may care for him/her; the best help you can give him/her is to (A), refuse to be a victim, and (B), direct him/her to professional help.
If you live together and s/he will not leave, you have to be the one to move out. This can be very difficult, especially if you have been cut off from your support system (friends and family) and have nowhere to go.
Not every controlling or manipulative person is dangerous, but some are. Most will respond to a show of strength - if you show up with friends or relatives to back you up, or if you refuse all contact, 9 times out of 10, this will be enough to make your point and put an end to things. If not, enlist help, either from police (a restraining order) or from a mental health specialist who may be able to help you identify whether your ex is a danger to you or others, or to him or herself, and will know the appropriate steps to take in that case.
Often, these people re-live significant events in their lives, re-writing along the way and changing the details to suit themselves. His/her recollection of conversations, impressions of you or others during the event, etc., can be terribly skewed, and can make him/her even angrier the more time goes by. If your ex is at all prone to violent outbursts, exercise extreme caution at any chance meeting.
If you have children, you may not deny your ex access to them unless there is a court order stating that you may. If your ex is simply controlling and manipulative, your goal is simply to protect your kids as best you can by pointing out the ways s/he may attempt to control or manipulate your children. If your ex is dangerous, and you fear that s/he may abduct or harm the children, you must inform an officer of the court, the police, or other help authorities of this so that protective arrangements can be made.
Don't assume that a mild, calm encounter with this person will end well for you; it may be weeks or months, but it's virtually guaranteed that you'll hear something horrible about yourself from a mutual acquaintance somewhere. Resist the urge to re-engage with your ex for the sake of "setting the record straight". Just let it go - the people who know you will figure out which of you is the more truthful, more by your responses and actions than anything else. Simply say, "That's just not true, but if it makes him/her feel better to say it... whatever." Just shrug and show them there's nothing you can do to stop your ex from saying such things, and then move on.
Watch for stalking or menacing behaviors, and if you notice anything, report them to the police immediately. This person is probably just difficult and not dangerous. But don't take any chances. If necessary, get a restraining or protective order and call the police each and every time it's violated; you will need the paper trail if the stalking escalates. S/he may try to destroy any action that indicates you moving on with your life, such as your career, a new relationship; s/he may deny access to your personal belongings or things that are important to you (which is why it's best to take them all and leave nothing behind when you go). S/he may even ask for a meeting to discuss re-paying you for credit card or other debt you incurred as a couple. This is a tool to maintain contact - don't fall for it. It may be an expensive lesson, but it may actually be better to just pay the debt off yourself rather than hook yourself into a payment arrangement that requires monthly contact to enforce.