First, thanks for having faith in a therapist by writing us here. There are so many things that seem off here that I wouldn’t blame you for distrusting the process. Please know I admire your willingness to take part by asking your question.
It sounds to me like there are a few things that happened that put you into a bad position to begin with. It your fiancé went to the same therapist for individual and group — then you joined in with the same therapist for couples — this typically isn’t good. If this is the case, this is not the usual way couple’s therapy begins because the relationship your fiancé has with the therapist creates a type of bias. If the therapeutic relationship involved individual and group prior to couple’s, then the deck is stacked against you before you go through the door. You don’t start on equal footing, there is a built-up relationship from individual and group, and there isn’t sufficient equality with the therapist going in. There are some unusual circumstances where this isn’t the case, but none of those conditions (like inpatient recovery) are present.
The second thing is your fiancé’s willingness to let you go. If the revelation didn’t involve informing you, bringing you in with a neutral therapist, and it was all acted upon without your presence, then he may have been using the therapy as a way to separate — not come closer to you. Of course, my comments are based on assumptions, but the whole process sounds off and this is my best guess as to what’s happened.
Finally, going forward I would ask that the two of you see another couple’s counselor who specializes in and is trained in couples counseling that neither of you know. The American Association for Marriage and Family Therapy has qualified people you can find in your area or choose someone from the find help tab at the top of this page.
Use this session to talk about what happened, and what the plan is the two of you need to have going forward. If there is any hope in reviving the relationship it has to be with a neutral therapist. If you need to end the engagement, using the therapist would be the healthiest way to do it.