You’re right. It’s very difficult to recover from a betrayal when you don’t understand what caused the betrayal to occur. A large part of the recovery process involves creating a mutual understanding of the problem — one where your husband is aware of the pain he’s caused you and you’re aware of the reasons he betrayed your trust.
My best advice is to talk to your husband about this issue. Tell him how important it is for you to understand the motivation underlying his behavior. Given that he’s doing individual counseling, perhaps this is an issue he can address with his counselor and share with you.
It’s also important to continue to appreciate and take note of the changes you’ve noticed in his behavior. Hopefully, your husband will eventually gain insight into what motivated his actions and be able to explain it to you.
I wish you and your husband the best.