You should never put yourself in a position where you find yourself dependant on somebody like this. She has a right to love or fool around with anybody just as much as you do. You let yourself be used when you let her start fooling around with you when you knew you'd break up.

I find a lot of self-hatered in OP because a person who truely loved himself and was secure with himself wouldn't of become affected like this. I believe the main issue is how you percieve yourself, that you may have low self-esteem, other than your affection for this girl.

It is hard, but you just have to forget about her. Trust me, I've been in the exact same situation.