Just being polyamorous is not being selfish. That's having the desire/capacity to love more than one at a time.
HOW you go about conducting yourselves inside you polyship configuration? If you are a hinge in a V turning a blind eye when you KNOW one of your V-arm loves is hurting? That's IS being selfish, because you are concerned more about your own well being than that of the others. I think you see this. So you are here struggling to make a choice you don't WANT to make but HAVE to.
If you have given it a good effort for a time? Done all the problem solving and all things are being done? You gave it a good try then. Did all you all could do.
Time to process results of the experiment? All players STILL not happy in polyship despite all the right things being done? Then this is not a polyship that flies well because one of the players is not thriving in a polyship configuration.
Maybe he is monoamorous and poly-friendly but THIS polyship is not doing it for him.
Maybe he's monoamorous and monogamous -- so NO polyship would ever do it for him.
Neither matters WHY -- what IS? Is that he hurts here. Alleviate the hurting. Time to land the polyship so this crew member can be free of this suffering. Land the ship.
Determine who goes where now that the polyship mission is over. You could make the call as the hinge to spare your loves having to make the call.
1) Break it up and stick with the marriage.
2) Break it up and stick with the BF.
3) Break it up and be alone.
4) Do nothing, just endure more of same. Til someone else other than you cracks and breaks it up. (<-- do not suggest this. It is not loving/kind behavior when you are already AWARE that your one of your loves is hurting to be turning a blind eye)
Sometimes polyshipping ethically is hard.
Could choose your hard.
I mean this kindly. Hinge role is not always easy.
But still... You see it. Don't ostrich. Could choose your hard. Make the call. In the name of love, you make the call.