I've read through this whole thread so far, so let me try to simplify the situation:
If you stay with this guy, then:
- You won't be happy if his wife stays in the picture.
- He won't be happy if his wife is out of the picture.
Unless one of the two parts of that conflict changes, it is obviously better for you to leave this guy.
So, do you think either of those two parts will change? Will you become able to be happy with his wife still in the picture? Will he become able to be happy with his wife gone from the picture? If the answer to those questions is, "No," then it's better for you to leave him.
I guess the problem is that you keep hoping he'll change, let his wife go, and keep you without that complication. He is seeing a therapist, after all. Maybe the therapist will convince him to choose, just you or just his wife. One thing to consider, though, is that the therapist hasn't convinced him of that so far.
What might be useful is for you to decide how long you want to wait for him to change. Not just, how much can you stand, but, how much is fair and reasonable? In some way, you should probably also account for the years you've already waited. Sure the circumstances have shifted to and fro over the years, but the upshot is that you've been made second chair, to his wants and to his wife's wants. Just think to yourself, "I've endured bad treatment for how long?" Figure it out. Figure out how much bad treatment you've endured so far. Then ask yourself, "How much longer is it fair and reasonable to ask myself to endure more bad treatment?"
Right now, he is treating you bad. He is asking you to live in a situation that he knows would make you unhappy. I think that is bad treatment. Why would he put you in this kind of predicament if he loves you?
I think it's pretty awful the times he's walked off on you and left you hanging. For how long? That's bad treatment. Is he going to stop doing things like that to you?
It might also help if you try to step outside the situation, as if it were not you in this situation, but rather, a dear friend. If a dear friend of yours was in the kind of situation that you're in, what counsel would you give to your dear friend? What would you want for your friend?
I don't want to try to tell you what to do. If you want to give this guy some more time for the therapy to take effect, and just to change in general, then I won't complain if you do so. But I would ask you to decide upon a very concrete timetable. Exactly how long it would be fair and reasonable to wait.
Try to be analytical about this, and have some mercy for yourself at the same time. How can you reduce the amount of suffering that you have to go through?
I hope things will improve for you.