My Metamour Betrayed Me

I had someone I considered a friend. We matched on a dating app so got closer and eventually became partners. I developed romantic feelings that were unreciprocated, so we decided to stay friends. Or so I thought. They admitted to feeling overwhelmed and said they didn't feel able to be my friend, but were interested in being my friend later. I told them that if they wanted to be my friend, they had to be my friend - but it could be a lot less intense if that helped. They said they can't be my friend and made some empty apologies saying sorry for the situation, for my feelings but not recognising that their choice to not want to do the bare minimum for friendship was what hurt me. Which extra hurt because of the way it reminded me of my abusive dad.

At the same time, my gf had matched with this person but conversation had fizzled out. But she restarted the conversation and they got together because she saw me talking to them. They're still together and they even saw each other after the person said they didn't want to put work into our friendship. Which hurts extra because it shows they can put effort into relationships, that they were choosing not to with ours. (note, this was during a weird period where person had said about not being friends but I hadn't understood and responded properly because I was very ill).

With my gf and this person still being together, I don't know what to do. I've decided to keep on moving on even if I feel so much pain from losing a friend. I did have romantic feelings for them yes, but this is almost irrelevant now because I'm demiromantic so with our friendship broken there are no continual longings to hurt me in that way. I still feel guilty and jealous about their relationship. Jealous with so much pain because this person is deciding to give my gf love and care but refused to give it to me, even in the way they ought to as a friend. I should be happy that two people I still care about are sharing that, right? But also I'm worried. Because this person, from my perspective they just decided to stop loving and caring for me, for an unfair reason out of my control. I don't want that to happen to my girlfriend. I don't want it to be my fault, for being the reason why they're together and not telling her well enough to stay away.

I want my gf to break up with this person. Not just because them being together causes pain as already described, but because I already told her the basics of what happened and if she can't recognise how agreeing to be friends then taking that away is a shitty thing to do, I don't trust her. I'd probably break up with her. But maybe that's a shitty thing to do? To need her to only be with one of us. The other person won't feel the same way, so from her perspective I'll be the needy and shitty one.

I guess the facts are, a person who has hurt me deeply is with my girlfriend. And that's the sort of mess polyamoury makes. Maybe I should put a brave face on it, it's not my place to interfere with their happiness. But maybe that's me not valuing my own feelings enough, and it's okay to say she has to break it off with at least one of us? But that feels like putting too much on her. Even if I do that, I must find a way to be kind. So if you guys could help with that it would be great. :)