I can definitely understand where you are coming from. However, I would probably have dumped him at the time he did it. You're worth more than having to put up with a guy who's obviously still not over his ex. To me, if you had threw things at him, dumped him, waited, and he STILL crawled back to you, it probably would have been a little more proving of his love than for him to cheat and still have a girlfriend with open arms. I know, I'm mean, but I honestly have no sympathy for cheaters. Drunk or not, he put himself in that situation. He must've known the girl was there and knew that you weren't (dont quite understand the situation), but still allowed himself to risk your relationship.

Now, that was over a year ago. You obviously have reasons to trust him because you've been with him since. I commend you for having the heart to forgive someone in that situation, because I don't. Anyways, let him know(if you haven't already) that you're trust in him will probably always be less than 100% because of that incident; therefore he will always need to step it up to keep you happy. If he's really as sincere as you believe he is, he will gladly meet the challenge imo.