Hi James,
I was in your position before and i felt a lil more insecure because stories streamed in about him when we were together.
I know it is a little hard to accept. But i learnt this: IF you can't accept it, then you probably can't accept her as your girlfriend. It is a fact that kept me in denial for too long. Things are clearer now. She had lots of bf but you have her now. Why focus on her past when you can plan the present and future. It is easier said than done. For me, i let myself lose him and i learnt this.
Whether it is too late, i am not sure. But if things were to work out, then i'll let it work out. If not, maybe we're not destined to be together. (That is how i see things though. Many probably think that i am too reliant on how things will eventually fall into place. I am not as passive as i sound).
Good luck. Just tell yourself that you've done something bad as well. Worse, it is when she is with you. Judging the unfairness of the situation, be honest with her and hope she'll forgive you.