I completely understand your emotions, my friend. I remember my first "real" girlfriend and I went down the same path. I wanted an honest and open relationship with her, which lead to her telling me things that would completely turn my world upside down. I was never able to look at a woman the same way, again.
Not to scare you, but this also marked the beginning of the end in our relationship. It would start with the knowledge and it didn't help that I knew some of the people that she had been with, which only made it easier for me to imagine in her in such dirty positions with them. It made it even harder to look those people in the eye, knowing that they had slept with the girl that I was sleeping with in the past.
The knowledge turned into disgust and partial anger. I couldn't help but see my girlfriend as a complete *****.
The disgust turned into jealousy and more anger every time I saw her talking to anyone who wasn't me, even if she was talking to another woman! I envisioned her sleeping with everyone that she talked to.
The jealousy turned into arguments and over the top accusations as I'd play private investigator. When the arguments got really heated, I'd make sure that she knew that I thought she was a dirty tramp. Needless to say, she broke up with me three days before graduation. I bawled my eyes out like a little princess and found sosuave.net in the process.
Looking back, the relationship was single handedly the best learning experience that I ever had. I learned that more often then not, women have a dirty past that we really don't want to know. Then, curiosity strikes in and we ask anyway.
I've learned to avoid the subject, period. It's none of my business and I really don't want to know.
Besides, it's the past. You know, she would probably have nightmares too if you told her every dirty little secret about your past, but we can't change our past. Therefor, the past is irrelevant. Try to concentrate on what's happening right now.
Don't let it bother you so much. It's not as important as you let yourself think it is. Concentrate on the moment.