Ah, thanks for the additional details. BTW, I think when you login, if you select that little check box that says "Stay logged in" or something like that, you won't lose your posts due to timing out while you're writing.
Anyway, what occurs to me is that perhaps you can commit to a time frame of six months or a year, whatever, within which you agree to monogamy and see how it sits with you to live that way. Maybe he will be amenable to a condition like that, rather than saying it's now or never. And then put your whole heart and self into investing in the relationship with a mono dynamic (no giving in to wistful hoping for poly) and see if you can be happy while monogamous. There will be adjustments and probably some good opportunities to observe and learn about your inner nature, thoughts, and emotional responses to things. I think that might be one way to know you gave it every chance you could to work on the marriage. You can agree to revisit the commitment at the end of that time and then determine if you absolutely must have a polyamorous life to be happy. Do you think that would work for you?