If it really bothers you, then I suggest you end things with her.
If it were me, I would be a little surprised about this information but it's not something that would bother me so much as to think of ending the relationship. I'll accept my partners past and appreciate the honesty. I'd trust that my partner has changed and will not be bothered by it.
She didn't cheat on you or anything, you said it's been great. It really just comes down to whether it's something you can accept about her or not. If you can't, be honest with her and leave.