I totally sympathize. I was the one who made the push for poly in my marriage, and I knew for years that I was poly but tried to push it aside because my husband wasn't comfortable with it. I don't know if it's possible to make that shift back to mono, but I do know that if I would have continued trying to repress that part of myself in my marriage, we would have ended up divorced. There is no doubt in my mind. I told myself I was "willing" to make the sacrifice, but sometimes "willingness" to sacrifice something is not enough to mediate the damage that sacrifice does to you and your relationship.
When we finally did open things up, I made it pretty clear to my husband that this was not a "trial period" that he could change his mind about. I will do everything possible to help him through periods of insecurity he might have, but that does *not* include suspending or ending my other relationship. He knows that if he couldn't handle poly anymore we would be done, not because I would be choosing my boyfriend over him, but because I couldn't possibly go back to monogamy and make our marriage work.
You *can* make it work with one of you poly and the other mono. My husband isn't entirely sure yet where he falls on that spectrum, but he knows that I will remain poly no matter what he ultimately decides is right for him with regard to other relationships. At some point, I think it becomes a question not so much of whether you're *willing* to live monogamously, but whether you really *can* have a relationship that is healthy for you under those circumstances.
Good luck working things out!