The short answer: You can’t.
Once a person cheats on you, a seed of doubt will be planted that will eventually resurface whenever that person exhibits even a bit of shady behavior, no matter how much you really want to trust them again. It's like dating someone who is cheating with you.
Even if that person leaves their partner for you, as soon as he or she does something shady (such as start coming home later than usual with no good reason or not wanting you to see who calls and texts them), you're going to jump to the conclusion that he or she must be cheating because they did it once when they cheated with you.
The saddest part of being cheated on is that it is likely to negatively affect all your future romantic relationships. For example, let’s say your ex had cheated on you with a coworker. In your new relationship, your partner starts to come home later than usual and begins to be less vocal about what goes on at work. Even though you might not want to think it, him cheating on you is probably going to cross your mind because you were cheated that same way before.