NONE of this is unexpected, remember? He hasn't been in a committed monogomous honest R in a long time, it takes time and hard work to get back to one.
What did you say to him when he asked if you could forgive him? Do you understand why he asked?
Now, why is it OK for you to go on and on about why you have to keep trying for Marc's sake, but it isn't OK for him to momentarily have the same feeling?????
He obviously cares for you. He wants to know if a good R with you is even possible?
"Yes, I believe I can fully forgive you. And, I believe you can fully forgive yourself if you get help. We BOTH need to treat OURSELVES and EACH OTHER with respect, love, and compassion. I believe we can start with a clean slate. But this is the deal. It requires complete honesty and openness, past, present, and future. Get all the skeletons out of our closets and put them on the table. It means an end to avoidance. Your continued contact with OW is what has started this latest mess. Rather than tell me what happened, you avoided the problem, hid your behavior and lashed out. Look where we are now. All contact must stop once and for all, officially. You must call her now on speakerphone and tell her that while I listen. It must be what YOU want. Moreover, there must be full transparency as we build trust. If you are hiding anything, any email address, and phone calls, any text messages, any visits from anyone from me, then that is a violation of full transparency. If we aren't in this R together to try to build it into a strong, passionate, trusting, fun, exciting, stable partnership, I'm not staying in it alone. I will never be more into an R than the other person again. Like you, I also need compassion, caring, understanding and forgiveness. I will no longer be in an R with someone who is emotionally unavailable to me. You need to do something to help yourself, you need to be a man who you like and respect, or you will never feel good. So, are you in this R with me to try to build something great or out?"