There are two helpful questions to ask oneself in the development of boundaries. The first involves asking yourself what you need to find safety and serenity. The second involves asking yourself what kind of person you want to be.
For those healing from Betrayal Trauma, one of the first things that needs to be assessed is their current emotional safety. If they are not emotionally safe, nothing we do in therapy will be very healing. This is one of the reasons I believe that couples therapy is often not useful very early in the recovery/healing process. The betrayed spouse needs to develop boundaries, and the betraying spouse needs to find some emotional stability and sobriety. Both of these are necessary for them to hold space for the third entity, the relationship, which includes the pain and issues both people bring to it. Individual healing and recovery must occur to the point where couples are in a space to work on the relationship. Therefore, couples therapy is indicated either when that individual healing and recovery is underway, or when there is a need for conversations as a couple in the development of safety and healing (such as working out the setting of specific safety boundaries, or determining together what “recovery” or “sobriety” looks like as defined by the couple themselves).
The analogy I use is that of a PTSD war vet. No one would ever conceive of convincing a war veteran to heal and work through his PTSD before he has left combat (technically at that point he wouldn’t be a veteran, but you see the point). The trauma is ongoing. There needs to be a level of safety present before healing can be attempted. For the betrayal trauma spouse, that safety is boundaries.