Fwiw, I've had two bi partners break up with me for women.
And I ain't mad about it. For me, if it's something that fundamental, as in them needing what being with a fellow woman provides, I know I can't provide that. I'm cis-het and male, so there's a wide range of experiences I have no access to, there's the physical differences, so getting broken up with on that basis isn't something that's on me. I didn't fuck up, it wasn't something I could have met their needs for and didn't. So, while it hurts, it somehow hurts less than a breakup for other reasons.
It's not the same thing as your situation, I know. I'm just hoping that maybe it can give you a different way of looking at your situation and maybe take some of the sting away, turn it down a notch.
Fwiw, I've also tried dating people that were poly, or in an open relationship (they aren't the same thing, but they can feel the same when you're a third wheel). It ain't for me either. Like you, what I really want is committed monogamy. It works for me. Which makes me agree that it's probably time for you to make that a hard no in your life. It isn't for everyone, and trying to make it work sucks for everyone involved when it won't.