How do I get past the past?

Cheater WILL cheat again. I think you knew that all along. In your mind, he did cheated on you. There is no solution as long as you are with him. He's damaged good now. What he does to you will always be with you until the day you die. You will NEVER forget it so don't waste time lying to yourself. Until you accept that, it's very difficult to build anything further. It takes years of therapy to undo the damage if you stay with him. Good luck