How can I forgive her (my ex-girlfriend)?

Honestly, she seems confused and feel you should move on... I mean come on all you are doing is setting yourself to continue to get hurt. Its easy to go back to your first love because they are familiar to you and why not deal with it so you won't have to start over... that is the wrong thing to do i feel you should just focus on you and transitioning to college soon i wouldn't worry about it plus if she went back to him so willingly then so obviously was never over her ex!!! : ) move on and find someone who won't continue to hurt you. but for your sake try and forgive her if you want its your call but don't settle because that is what it seems like you would be doing. just leave her alone for a while and seem what happens