I agree with everything said here except the people who said not to bother with Bipolar people. Maybe I don't agree because I'm bipolar. *shrug*
Anyway, she probably cheated on you. But if you want her back, treat her the way she treated you. (Like you don't want to see her often; you're better than she is, etc) I acted like this with my boyfriend and he finally ended our relationship, which I thought would never happen, as he was pretty obsessive about me.
When he did end it I found that I was totally crazy for him. All of a sudden I wanted to hang out with him a lot and get back together. As he still didnt want to, the feelings were even more intense.
The point is that it will take a lot for the girl to forget about you. Unless another great guy wanders into her life and distracts her, she'll be stuck on you. (And even if she is with a guy she'll want to cheat on him with you, probably) Just don't blame her for being cruel to you sometimes. She probably can't help it.