Her past is just that; her past. Everyone does things that they later regret in life. Maybe focus on the fact that she chose to trust you with the truth, even though she only shared because you wouldn't let the topic rest and hadn't trusted her initial response (even though she'd initially lied, you had no way to know that...and the lie sounds like it was because she seems ashamed of it).

The opening of communication about the past can really have potential for helping your relationship grow even closer.

And when you think about - you said that you can't even remember how many people you've had sex with. Which means that some of those people were so insignificant to you that you couldn't be bothered to remember them. What sounds more shameful to you - having a higher number of sexual partners at a young age, or having sex with so many people that you can't even remember? Personally... I'd rather have the younger number and remember everyone I've had sex with. Comparing that in your mind might help you be more accepting of her past.