Welcome to SI.
I feel great empathy for you. All betrayal hurts, but when you have spent decades with someone, raised children together, and faced tough things, the betrayal and recovery has some special dimensions. I agree with the very wise Sisoon; part of this was some rugsweeping. Yet I am here to tell you that I did not rug sweep. We read books, did the work, learned through cognitive behavioral therapy to change our patterns. We are doing well in reconciliation, and yet there are times when I am still so, so, so sad.
In your profile, I hear some things that sound similar to me. You have a lot of empathy for the pain turned a person your loved for decades into this stranger that betrayed you. You also have anger and hurt, because even before you knew the affair was the reason, you became the villain in his head, and you felt every one of those thought distortions like a knife. Then, then... you learned that the reason for this transformation was his own guilt and shame over the affair.
That means you have a dual perspective; you see all of your own pain. You see his. If you are like me, and way, way too empathetic, you even see the APs pain. It means you are capable of forgiveness, and that is a beautiful thing. But it also means that there is a tremendous risk that your own pain remains invisible to others because they really are not capable of seeing what you see.
Yes, therapy can help you learn to set some boundaries, but know that you are not alone.
Next time you are feeling alone, remember that it takes time. Remember that you can and should talk to him about what you need to feel safe--not just from an affair, but safe in revealing where you are still broken so that you can heal. Remember you can come here and talk to people who will understand why you still feel broken.
Something did break. Something was lost. It was like a wild fire. Everything has to regrow, and that takes time.
Just know that you are not alone. I get it. So do many others on this site.