How do you know when your marriage is over?
This is a really rough rut to be stuck in. This is a perfect example of why couples therapy is often a necessity.
It sounds to me like you're stuck because you're torn between feeling a sense of commitment, the needs of a child and your feelings of betrayal. I don't know if you still love him, but that often is in the mix somewhere as well.
In therapy, I often find people feel liberated to talk about core issues in a more deliberate way. The platform has a lot of "worksheets" as well that provide an informative way for both partners to look at the self, and gain insight into better ways of interacting that encourage more openness and warmth.
However, the hard question is, "Should I end the relationship?" If you've stepped over that threshold, then therapy probably won't help heal it.
It's a good question by the way, and a very important one. In the case you made the decision that you're "done," I believe individual therapy would be crucial. You would want deeper knowledge about you, your needs, and how to proceed without undue harm to your own being, or that of your child.
Therapy is a way to explore deeper issues, whether it be to enhance or repair a relationship or an individual. Regardless, in effective therapy, the individual gains insight and personal strength.
I tend to focus on personal empowerment. Part of what I do is to look at trauma issues that might exist in a person that could contribute to a sense of helplessness and hopelessness. I base this on a theory I call "Displaced Power."