Only you know what you need to do to sleep at night. If you can't let it go, you're going to have to talk to her. But I'd be careful about how you put it, "I don't trust you" is a pretty big bat to swing at someone you've asked to share your life. You can do damage with it.

But let me ask a different sort of question (ones I don't want or expect the answers to, just things for you to think about)....are you the sort who often suspects his girlfriends of sleeping around on him? Was cheating somehow involved in your divorce or in previous breakups? Is this the only reason you have to doubt her? Because if you're piling up some yes's here....you might be projecting doubt onto a perfectly solid relationship....and you might need to do some work on your own place in the relationship. It's not entirely uncommon for people who have been burned before to develop "sudden suspicions" as a way of distancing/protecting themselves from deepening feelings/vulnerability. Not saying it's true, just a thought.