JasonR,
I'm in a similar situation. I've dated my girlfriend for 2 years now and at the beginning, I had to go to battle with myself in order to deal with those thoughts about her past bothering me. There are a few things I learned that I would like to share in the hopes that they help you wrestle your way out of the problem.
First, with all due respect, you CANNOT expect insight that a woman gives you regarding this problem to be of much help. I don't mean that in a sexist way. Rather, this is a guy problem and something women just cannot seem to understand. I know this because I talked to a number of women about it and each time, there seemed to be a gap between what I was feeling and what they were understanding about the problem.
Second, those feelings you are experiencing are a reaction of inherent male attributes. Namely, those attributes are a male's need to feel special insofar that he is the only one that has ever seen his girl naked, his possessiveness (as in "no other guy should have had fun with my girl"), not wanting to feel sexual inferior or less experienced than his girl, etc. So in other words, what you are feeling is totally normal. Don't let anyone make you think you are being unduly possessive or insecure. In fact, you as manly as you can be.
Third, I found that it was helpful for me think about it in general terms. For example, if you knew a guy and his girl had the past that yours did, would you think of them differently? And if so, how?
Fourth, you should remind yourself that life is all about tradeoffs. When we are young, we want it all. Personally, growing up I imagined I would marry the perfect girl: she'd be a virgin, would have never come into contact with another guy, etc. But as a grown up, I came to understand that life isn't perfect. Same goes for this. Is your girl's past so burdensome that you would let go of what you have now? If not, its time to be a man and bite the bullet.
I hope this helps. Just remember, a woman wants to be her man's last love; a man always wants to be his woman's first. This is a natural phase and it too shall pass. Good luck, keep fighting and don't give up on her too easily if she's worth it.