She's very insecure. She is making you pay for the mistakes of others, including her own for dating them. Be very clear about this though: its not about you. Its about her and her inability to trust.
She needs to let what happened go. Yes you screwed up, yes you are sorry. At the end of the day, you want to be with her. If that isn't enough for her, then I agree with Stung. Break it off. Kick her in the ass to take the chance it will screw her head back on straight. No need to be an ass about it, though. Just say its too bad she can't trust you, which is HER problem, and you can't be in a relationship w/o trust:
Trust = comes from her
Trustworthy = comes from you
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh