I had this girlfriend that told me everything about her past (fuck, suck, fuck with other men even while she was pregnant and in a relationship). Her lies, how she deceived and used other people, she told me everything.
I made her told me.
I felt better. Even with all those things she did, I realized that I still do love her. It's better to know everything about the person you love.
If you really love her though, be contented with what you know. Don't force her to spill out her past, she'll be bound to tell it anyway (or you'll eventually find out).
Please don't tell me about that "past is past" bullshit, what you did in the past affects your future. If you were a slut, don't expect to find a virgin even if you've changed for the better, it's just unfair with what you did in your past. What you did is what you did no matter what excuse you make.