First of all, sorry for your pain. Here's my general theories:
1. You're picking the wrong type of guy, and probably doing it over and over. I've found that there's frequently a large disconnect between who I find "attractive" and who I'm actually compatible with. As you look back, do the guys you date have similar qualities/personalities, etc.? It's easy to be drawn in to a certain kind of look you like. But there's no guarantees there.
2. You're *possibly* too needy. Obviously, I don't know you at all. But don't throw yourself at some dude and tell him he's your whole world. Have some mystique about you. Have some hobbies or things in your life that you like and can do that will tell him you're perfectly fine being an independent woman who doesn't *need* a man to fulfill her. I'm not talking about teasing. I'm talking about not being available to him to go out whenever HE wants to do something. You have to show him (not tell him, but show him by your actions) that he can't take it for granted that he can just call you up at a moment's notice and be at his beck and call. That's too easy, and it's boring. Plus, if you have interests other than him, you will have more personal satisfaction, anyway. I think a few "Gee, that sounds great, but I already made plans" responses (even if you haven't) when he calls at the LAST MINUTE to do something will work wonders. He'll be more interested. If he's not, he ain't worth your time.
3. ALWAYS have self-respect. The moment someone even hints at being abusive or disrespectful, you need the strength to end things. By the way, here's a hint: It's almost always easier on the person doing the breakup than the one being dumped.
That's my Dr. Slikk for tonight. I've been there (and I'm speaking as a guy). Getting hurt sucks, and it really is like getting punched in the stomach sometimes. I had to take a good long look at why I was choosing the same kind of woman to pursue and date. There is an old expression that goes something like this: The definition of insanity is doing the same thing over and over and expecting a different result. That's what dating can be like. You really need to take stock.
One last thing: If you have girlfriends who can actually be honest with you, maybe you should ask them (without penalizing them for their honesty!) if they can see anything about you or your personality/mannerisms that could be a turn-off to guys. Or, ask a guy friend. Remember, though, hearing nice things about yourself from people who don't want to compound your pain isn't going to help. You need the truth, or at least from their perspective. Then, take what they say and evaluate it. It doesn't mean you have to accept it, but just evaluate it and see if you agree.
Good luck. Now, go have some ice cream.