I can understand the emotional agony. If you are back with you, I would assume you still love her, but sounds like you have not fully forgiven her yet. That would explain the insecurity. Sitting her down and communicating your feelings would probably be a good method. This will let her know your position and also let her make some choices “if” this other guy is still around. Trust can be gained through reassurance, but as you progress… try not to treat her differently or assume she’s always cheating. In other words, just love her the best you can.
Personal thought, if she had an affair and you two separated.. that other guy was still probably around. If he was that special, she wouldn’t be with you right now.