The fact is, and I'm sure many people will back me up here, your first love is different from any other. Most people enter into their first relationship recklessly, with no thought of getting hurt - this makes it more comfortable and intense than any other love afterwards.
This is especially true if your first love cheats on you - when you start your first proper relationship there's no reason not to trust the person 100 percent, so if he cheats on you, that's a huge blow. How can you ever trust anyone again? That relationship affects every single romantic encounter for the whole of the rest of your life, in one way or another.
Don't give your girlfriend a hard time. She's been honest with you, which is great - don't give her the impression that this was the wrong thing to do. Although she finds it hard to trust you, this isn't because of the way you behave - it's all wrapped up with her previous relationship. If she was really hurt, the fact that she's got this far in trusting someone else is really good.
If it's not too weird for you, try getting her to talk about how she felt when she split up with him. Try getting her to vocalise all her feelings about how much of a bastard he was - this will help her, and will give you a bit more information abut why she's the way she is.
I think the main issue is the way you deal with the situation. You could easily let it bother you too much, accuse her of wanting to be with him, and get insanely jealous, until she doesn't want to be with you any more. If you can try to get your head around it and talk it through, you will come out the other side with a mature relationship to be proud of.