My partner and I have been dating for a year and we both realized we are polyamorous some time ago. Some months ago, they started dating this other person, an older girl she met online. I was OK with it but I insisted I meet this girl. We got close, in a way, and I wanted that, I needed to trust my partner's partner otherwise I'd feel wrong with the whole deal. As I got to know her I realized she's a toxic person, she's possessive, overly jealous and plain rude at times. I can't even talk to my partner on social media without her throwing a tantrum about "not being loved by (my partner)". In addition to that, she has misgendered me a couple of times (I'm a trans boy) and done things that make me highly uncomfortable (and I've told her those things make me uncomfortable but she keeps doing them anyways). How do I talk to my partner about this? I am afraid to do so because they might think I am just not okay with the poly. And honestly I'm not too sure I am but only if they have a girlfriend as possessive as the one they have right now. I feel constrained and I am afraid they might hurt my partner.
It’s okay to just come out and tell her basically what you told me: “I have some concerns about how your girlfriend is behaving - it’s negatively affecting me. I don’t want you to think I’m not okay with polyamory in general, but polyamory with this specific person isn’t working for me and I need to talk to you about it.”
I’d advise against coming out and saying that this person is toxic, possessive, etc. - that’s too subjective and opens you up to unwinnable arguments. Focus instead on specific behaviors and examples. “When she misgendered me, that was painful and not okay, and makes her an unsafe person for me to be around. When I asked her to stop doing [thing] and she kept doing it anyway, it made me feel like my boundaries were being violated and that I can’t trust her to take my feelings or needs seriously.”
Be clear in this conversation about what you want from your partner: do you want your partner to stand up to the girlfriend more and say “hey, you can’t talk to my partner like that, please stop”? Do you want to spend less time interacting with this metamour and need your partner to help facilitate this space? Have you realized that you don’t want to be in a polyamorous relationship that includes this person, so you’re letting your partner know you’re going to have to leave the relationship as long as she’s involved? (Note that this framing is very different from “dump her or I’ll leave you” - you’re not making demands or threats, just identifying the right choice for you to make based on whether this situation is healthy for you to be part of.)