You ask a very good question and a very difficult one to answer. When somebody cheats on us and we decide to take them back we are saying that we forgive their act and will do our best to not let that get in the way of our progress. It does not mean that you are required to forget it happened.
I think the only way to heal is with time. And open dialogue. While you can’t beat him over the head with it every time you get upset about the fact that he cheated (as that’s not really forgiveness), you do have the right to let him know how hurt you were by it. A lot of people who get in trouble think that you talk about something once and that’s it. They believe that if you bring something up again then you truly arent over it and you’ll never forgive them. It’s human nature to want to move on past something we’ve done wrong as quickly as possible. But it is unfair to expect somebody we wronged to never mention it again.
So I say that the best way is to probably just take some time to yourself when need be and really make sure that you are okay with being with somebody who hurt you as much as he did. If after thinking and reflecting, you are, then you keep moving forward until the next time. There are no statutes of limitations on how long you can feel some kind of way about anything. Take the time you need.
Also, make sure that he’s doing what he needs to be doing to show you that you can indeed trust him again. While healing is often a personal thing, when it comes to being wronged by another it is alright to expect them to make changes that would show you their intent on being faithtul.
It’s not an easy situation, but it is one that many people have dealt with and moved on from. Maybe you can; maybe you can’t. Only time will tell.