I have no good advice, I just wanted stop in from the all feed (I'm poly by practice for the time being but it's not really the way I intuitively relate to relationships or what I want long term) to offer hugs and say I'm sorry you're in this incredibly stressful spot, I'd be so afraid it'd all end with people getting hurt
I'm hopeful you find the right approach for your situation. At the end of the day if the people you're interested in are monogamous and do not wish for a different structure of relationship, you may be forced to face simply not being compatible and wanting different things even if you're in love with them both :(
Sending love and hugs 🫂 best of luck my friend. Remember to be compassionate both to your partner(s) and to yourself.