Unfortunately it is an excuse. Simply because he is not ready to let go. It is simple. He needs more time. Give it to him.
I know you want to work on the M. You, like all of us, are ready for it to be over. I tried to force my H to end the first one and the second, just left the third one alone. It didn't work and made me the bad guy. I look back on the first one now, ten years later, and I have had to wonder why I put up with it for two years. Why I put my mental health and my physical health at risk. But the reality is I did it and I welcomed him home with open arms. But I never got rid of her, until this all started again. I'm sure that my feelings about that and the way I handled it (or didn't really), have impacted where we are now.
So suggestions? Give him what he says he needs and wants. Until he is really ready, anything else won't help. And by trying to force it, you will always wonder if he really chose you.
"Acceptance doesn't mean resignation. It means understanding that something is what it is and there's got to be a way through it."--Michael J. Fox