I'm gonna follow the mass here and say that some things are best left untold. I found out the hard way - sort of. My girlfriend only slept with one other person before me, and she was my first. Just thinking of that guy with her makes me sick to my stomach now... I know the guy, and hated him before I knew they had sex, now I have nightmares about this guy and her, it eats at me every living moment - but the fact is, I love this girl more than anything, and willing to let her past go, and would be very willing to have never found out about it anyhow. What I guess I'm trying to say here is the same thing everyone else has said: forget about it, you don't need some mental picture sticking with you for the rest of your life that you know to be true - it will be far worse than whatever you've conjured in your mind, for the sheer fact that you know it to be true.
Good luck, and hope everything turns out okay between you and her.
