"I'm afraid I am kidding myself. I don't like that feeling."
Then lay out your ground rules with him. Tell him honestly that you are only going to be going out with him if he is not having any contact with the OW. Tell him that you want to trust him and you're going to give him the chance to come clean and will forgive him for it, but after that, there will be no second chances.
After that you are going to have to trust him at some point. If you can't learn to let it go, it will always be on the back of your mind and even if you do reconcile, you will always feel that you can't trust him.
M-43 W-40 2D - 9 and 5
Emotion, yet peace. Ignorance, yet knowledge. Passion, yet serenity. Chaos, yet harmony. Death, yet a new life.