Thank you, LacunaCoiler.

In a surprising twist, the co-worker (G, for the sake of ease) sought me out last night to talk things out. Turns out I didn't need to deliver an ultimatum. Not that I probably could have, despite the advice you've provided.

G was worried about me, and has been suffering due to this heavily K-driven situation as well. We discussed for a long time. Then G privately ended things with K.

I comforted a profoundly upset K. It hurt. She said some hurtful, spiteful, selfish things, but I know this is painful for her, so I showed her the compassion I wish she had the capacity to show me.

She wasn't entirely oblivious to the hurt this caused me, and did apologize, though the apology came off a little mean, sort of a "I'm sorry you can't tolerate my needs" deal.

She repeated immediately that she still believes she's polyamorous, and I confirmed that I'd support her, but that we both should talk to other poly/mono people before trying another relationship again, because as of now, I can't emotionally handle it.

But that's OK, because K said she couldn't imagine meeting someone as special as G again, so why bother?

So... was I not special to her? When she proposed to me? When she married me? I'm just a villain to her now, holding her back from what she needs due to my selfish, possessive whims.

I don't know what to do. I feel so stupid. I'm just as brokenhearted as before, and the future looks bleak for me, the deadweight wife who took someone special away from my spouse. I don't want to leave, but I don't want to stay. I mostly don't want to exist.

I'll find a way to cope. I'll shut up about it here. I feel guilty for taking up people's time only to ignore good advice. Thank you again, everyone. I think the saga has concluded.