Commit to Full Honesty

If you even got to the point that a partner could betray the other, then a serious breakdown of communication had to have happened. You need to BOTH fully recommit to being fully honest with each other. One natural result of a betrayal is that the partners don't really trust each other afterwards. "If he could lie about this, what else could he lie about" is a common thought.

So be fully, openly and extensively honest about EVERYTHING, at least for a few weeks. Share every feeling. Ask about every thought. Don't worry about information overload. Just keep sharing. It might get to the point eventually that you can start to back down on the level of sharing that is going on. But to get through the initial worries and hurdles, be overly sharing, constantly. The more you are talking and sharing and knowing EXACTLY what the other feels, the less wild leaps of logic your minds will do.