My girlfriend can not let go of the past!!?

Wow she can't possibly be mad at you for that, #1 you weren't together, #2 she went and slept with someone else, so it is YOU not her who has the right to be mad about that. I think you need to TELL her how much that hurt you, and you need to explain that she has no right to try to rub that in or bring it up! because that is a double standard