I echo hamster-bamster's question.

I will also say that trust, once broken, is really hard to get back. I still don't completely trust my current partner, even though she's done nothing wrong; just the opposite, in fact. But I am so used to being with people who are inappropriate for me and who were dishonest and so I projected that onto her. I am just finally getting to the point (a year and a half later) where I don't freak out or turn cold every time her text messaging goes off. It still gets my back up to see her on facebook.

Do you trust him? Do you believe he's telling the truth about not sleeping with the other girl? That's important. Because whether or not he did, you have to be honest with yourself about whether or not you will be able to trust him. And at some point, whether or not you ever get a satisfactory answer from him about anything, you have to be prepared to make a decision. It sounds like you're quite unhappy in this situation, and usually checking up on someone is not a very positive sign for the relationship (not intended as a judgment on you).

Regardless of what happens, I wish you all the best. I know it can be difficult to be in these situations and I hope you come to a place of peace with it.