How do I get over it?

So about 5 months ago or so, when my bf and I had been together for about 4 months, he dumped me. I later found out that he dumped me for another girl. This girl, let me tell you about her. She is in the same class as he is (one year above me) and she is really pretty. She has bright blue eyes and perfect skin and perfect hair. She always wears cute clothes. She might not be the skinniest, though that doesn't take away from her beauty. Before all this happened, I never really paid attention to her, but I didn't necessarily like her. I did think she was annoying and complained a lot, which she does. But back to the story, I suspected that something had happened at her party that she invited my bf to. I didn't realize until after it was too late how much she liked him. Nothing reallyhappened at the party, but Im guessing there was a lotof flirting. Then he dumped me the next day, saying he didn't love me as a gf anymore. I made a big deal about it. The next night he apologized, saying he didn'tnowhat he was thinking. I took him back. A week later, hedumped me again. 3 days later, he apologized even more. I could honestly tell it was sincere. And things are perfect between us since then, we havent been fighting and we haven't broken up or anything. and the girl hasn't been causing problems anymore than usual. But ever since then I have felt obsessed with her. For a while I tried to copy her style and be more confident like her. Eventually we made friends. Now I hate her again, be ause I realize that I never really liked her. But still, I find myself searching all over facebook for pics of her and asking our mutual friends about her. I can't let it go. And every time she is around, I make fun of her and laugh and stuff. I don't know why I can't jut forgive and forget. I keep having nightmares about she and my bf kissing I feel bad because my bf asks why I'm sad and I have to Keep telling him about it and I think he is getting annoyed and I feel bad. How do I let it go?