Hello AnotherConfused, I am also poly dealing with a mono life partner, and even though I don't talk much on these forums, I feel like your story resonates with me a bit and thought I might be able to offer some advice. (You can decide whether that advice is good or bad.)
When my wife and I were trying to bridge the gap between us, things got really tense at some points. I was adamant that Poly was not something I was willing to ignore. There were times that I felt very guilty about the pain that I was causing her, but this isn't something that I could put to the back of my mind. I was in love with a second person already. It can't be undone. Being capable of loving more is something I was born with and I refuse to feel like I am a bad person because I have more love to give than most.
Of course she was perfectly within her rights to not be poly also.
We were stuck. There were fights. They were really bad sometimes.
What it came down to is that we had 4 options to choose from.
A) You both compromise a little. Make it work. Stay together.
B) I give in completely, we are monogamous, and I suffer.
C) She gives in completely, we are poly, she suffers.
D) We admit that it can't work, and we find people who are more compatible to us.
Once we sat down and talked about it, we realized that options B & C weren't options at all. We were both too stubborn and willful to break our egos like that. Option D seemed really tempting sometimes, to both of us, but we never pulled the trigger. What it came down to was that we loved each other. Because of our love, we endure the pain that we have to (both of us) in order to live with each other. Option A was the only option for us.
So now you have to ask yourself these questions:
Are you willing to suffer through an unknown number of years of monogamy? (Option B)
Or are you willing to leave your husband shattered and in emotional turmoil for an unknown number of years? (Option C)
Are you willing to walk away? (Option D)
If aren't willing to do any of the above, then you don't have much of a choice either, just like us. You've got to dig in your heels and fight to make it work. Talk, talk, talk and when the horse is just bones and a saddle, beat it some more. The two of you will fight, and it will hurt. You will cry, he will cry, your lover will cry. Someone will probably threaten to walk away, but you just keep on fighting till it's over.
If you survive, the two of you will be closer and stronger than any relationship you've ever seen. Just be aware that not every relationship makes it.