Has he ever expressed to you that he needs were not being met? What are they? Have you made any attempts at being a better wife to him, in the way he has requested?
I guess I'm not understanding why he wants to leave so bad. And why you are so desperate to prevent him from doing that, and yet so darned unhappy that things are the way they are.
Is he not giving you a fair chance to right the wrongs of the M, and just saying, Either sleep with another man or we're calling it quits?
I'm with everyone else. Something is not right here. I don't understand why he wants out and why you want him to stay so bad.