I've spent a long time considering if I'm polyam and I believe I am. It makes sense to me. However I've been in a monogamous (F/F) relationship for 6 years. We live together. She understands polyamory, but does not want it for us. When I tried to bring up my self-discovery, she became verbally abusive toward me, threatened to leave me, and insinuated she would probably hurt or kill herself. I'm afraid of destroying our life together over my feelings, but I also want to be true to myself.
Leave this relationship. Someone who becomes “verbally abusive” and threatens to hurt themselves as a way to control your behavior and feelings is not safe or healthy to be with. I know when you’ve been together for so long and you live together and have built a life together, it’s really hard to consider leaving, but when your partner acts like that, it’s time to hit the emergency eject button.
IF this was completely out of character for her - IF she has never, ever acted like that - it might be worthwhile to try and have one more conversation where you explain, very seriously, that you are now really scared for her, and for you, and for your relationship. That what she said was absolutely not okay. That you would like to get some therapy together - not with the goal of getting you a ‘free pass’ to date polyamorously, but to address whatever underlying issues caused a vulnerable, difficult conversation to turn dangerous.
But if she responds with anything other than “oh my God, I can’t believe I acted like that, I’m so sorry, it was not okay - I still feel strongly about the polyamory thing, but I need to find a way to have strong feelings without becoming threatening and abusive” - you need to leave. This is not about polyamory, this is about you being with a person who is currently unsafe.