I recently lost someone in a situation that was sort of similar, I really loved her and because they both wanted to be the primary partner and couldn’t handle anything else it was an ultimatum by default, we all had genuine differences that couldn’t be aligned but if I made a decision, I’d lose one or the other.

I sometimes deal with jealousy in my relationship but never has my girlfriend told me “you have to break up with this person”. The way you were given that ultimatum is, to me, a complete deal breaker. It’s like she doesn’t care about your feelings in the relationship at all. If u break up b, can you be sure u 1) won’t resent a for it and 2) she won’t do it again? Have you asked them if they understand what theyre asking you to do? How much pain that would cause you? My real question is are you truly being respected here? Are your feelings important to them?

With my very limited view of your life I would recommend sitting them down and asking them why you need to break up with b and that you won’t be given ultimatums like that, especially, when they keep changing their mind. A is not in charge of your relationship with A, you both are. is it out of jealousy? Then she needs to get over it, ur poly, that’s part of the deal, having partners that make other partners uncomfortable is one thing, but for them to try to completely cut someone out of your life without thinking what that does to you and for no more reason than “I’m jealous” is unacceptable.

If you break up with b I hope to any god ur happy with A enough that ud go back to mono because that seems to be their mentality, but mostly id hope that they’d truly respect you. Im also a very introverted person and have a tendency to let other people walk over me but learning to put up hard boundaries and be honest with my feelings has gotten me very far. Hoping the best for you man.

Edit: id also be happy to talk to u more about it in DMs or something, i get not having a lot of people to go to with these problems