No Lemonade to be Had There!
I'd say you picked a lemon. I've gone through this exact scenario, and find that people who have been mono most of their lives have a hard time wrapping their heads around poly, or even seriously considering integrating it into their core.
I have found that releasing attachment & expectation, slowing down pacing, and exploring connections with more than one person has been helpful for me. When I pin all my desires on one person it's way worse than if I'm lightly dating a couple of people to see if there's true potential there.
I have been in that exact same scenario, and had to step away from it and move on for my own happiness, no matter how into him I was.
Where you go... there you are.
Me: 35. TD, 43, my monogamous beau. Lily: 31, my lady/lover, in two other relationships. Mahogany: 38, my girlfriend, in one other relationship. Elemental: 44, my ex husband.