You can't fully. He will always doubt you. I know because a guy cheated on me and, no matter what he said about how it would never happen again, it was always in my mind when I hadn't heard from him or when another girl looked like she was close or all jokes with him.
But you can try and move on. The only thing that will do that is time, and good times. When you start to get back into things and start having fun like old times, then it goes to the back of your mind more.
