I think i'm still in love with her, what should i do?

You need to move on, maybe it would be best if you didn't even try to stay friends with her because it might just hurt and remind you of how you felt about her before. She obviously didn't feel the same way about you because she stayed with her boyfriend and if she was willing to cheat on him, who says she won't be willing to cheat on you if she finds another attractive guy? I think you should move on, get to know this new girl, maybe she will be a better experience for you. Everybody goes through that first love experience and sometimes they never really get over their first love completely. But you can move on. I did, and I know I will always love my first love (we are still good friends) but I am IN love with someone else, and I am very happy this way. Good luck, hope all works out for you :)