Hi Silverbirch, I'm so sorry that you are going through this, I know from personal experience how hard this is.
I too used to say that I would never forgive my husband if he did this to me but when after ten years of marriage he did, I found that it was more complicated than that.
I somehow found the strength to stay calm, somehow decided that I was not in a state to make any decision that would shape the rest of mine and our children's lives and just took one day at a time. I could see that he was miserable in his decision, truly miserable, and I came to believe that just because you do a bad thing does not make you a bad person. I'm not a fool, I know how fundamental this "bad thing" was but I loved my husband, my children loved their daddy but when he unexpectedly asked whether I could ever allow him to come back I found myself unable to say "no" and saying "I don't know, maybe" and giving it a go.
It was hell for us both for a while, I was angry for a long time. I didn't trust him for a long time. Counselling helped. Seeing his regret helped. Time helped. Keeping my family together helped. There was a before and an after but 15 years later I can honestly tell you the after is much much better. We don't take our marriage for granted, we don't take each other for granted. For us, I'm grateful that we took all the effort that we might have put into a new relationship and invested it into the relationship that we already had.
I've found it very hard to write this, but just felt that an alternative perspective might be good for you to read. My only advice would be take a breath, try and stay calm, stay focused on what's best for you and you will figure it out. Good luck!