How can I forget about the past and move on?

my name is amy and i am a highschool student. last year i got a new boyfriend named derek. we went out for seven months and broke up during the summer for two months.

right before we went back out, he liked my used-to-be close friend sarah out. after she said no he started liking me agian and two weeks later we were going out. a month later he told me that he still liked sarah. at that time derek and i werent as close because we wouldnt talk a lot at school and the only thing we did was talk and nothing more. i found out he liked sarah because he told me he was thinking about breaking up with me but one night he thought about it and decided that he had everything he needed with me. we fought for the next couple weeks because i was hurt by what happened. during those couple weeks i started to have a little crush on my close guyfriend but i think thats because i wanted to get back at derek for what he did. after that he stopped likeing sarah but would still talk to her a lot and flirt with her. this went on for about four more months. now 10 months later we are still going out but i cant help but think about what happened for those months. it really hurt that he could like another girl while going out with me. we are now so much closer and he has changed a lot but i still think about it and it still hurts. i now hate sarah and we dont get a long at all. even when derek and sarah are in the same room i get jealous because im afraid he could like her again. he says he doesnt and he thinks shes ugly now but im afraid he could change his mind. could anyone tell me how to forget about the past and move on?