My advice is that you are going to lose this girl for sure if you don't start trusting her.
Lets say she is being totally faithful to you - why on earth would she put up with all of your accusations and mistrust?
How long are you going to feel like this?
If you are going to feel like you can't trust her forever then please just put this relationship out of its misery - because you are going to be miserable and you are going to make her miserable.
If you decide you can trust her - then just trust her. A relationship is not complete if the partners do not trust each other. I don't really see that she has done anything particularly shocking in her past. Find me one girl who didn't kiss an older boy when she was 15 - or at least *want* to kiss an older boy when she was 15 - Most girls have, I think. We're talking about kissing here!
Lighten up, man! What does your track record look like with girls - is it as pristine as hers is with guys?
As an aside - I would suggest in the future not to go prying too much into previous relationships with your future GF. It's one thing to ask if somebody might be at risk for having an STD - I can respect that (although you should protect yourself regardless). But we all have a history by some point - whether it is a prior relationship, secret crushes or wild escapades- and you are looking for a fight, hurt feelings, and mistrust if you start prying too much. Respect your partners privacy and her past. This particlar GF of yours sounds like she has been very honest with you -- and look at the price she has paid for doing so!
Anyways - sounds to me the problem lies with you, not with her.
I'd seriously think about some counseling - because I think you have some trust issues that have to deal more with you than with your girl friend - so do both of you a favor and get some help.
