I had until recently a relationship with a partner, we were together for 1,5 years. He was wonderful, fun, loving. He lives in a different city but would come to visit me every other month. He has another partner who he has been with for a little more than two years. Last fall she moved to the same city as him.
We have regular check in meetings when we see each other usually just to bring up anything that we might need to talk about.
When he was here the last time he told me about a situation with is other partner.
Both of them had been dating other people as usual. But she had now told him that it didn't feel right to her. She asked him why he needed to date other people, that she thought that they had a primary relationship (although he said to me that there was no hierarchy), and basically asked him to choose between her and I. He had said that he couldn't do that because it wasn't fair to me, and she then said that she was out. He was devastated about this, and when he told me I could hear on his voice that he was deeply hurt and crushed by the thought of having lost her.
He told me that he didn't know what to do, that perhaps he needed to take some time from all his relationships to think about why he was polyamorous, why he seemed to be attracted to a certain kind of person, that he didn't know. I said that I wanted him in my life always no matter in what form.
Then he left to go home. When we called each other about two days later he told me that he wanted to deescalate our relationship to a friendship, he said that "he had to do it", that he couldn't loose her, that she needed him. And that was it.
I feel thrown away. I feel like even though he said he loved me and that I was important that I was actually disposable to him all along. I feel like he never saw me as a person. Even now. He got what he wanted and I am sad and alone. I think he isn't even sad that he hurt me - why would he be? He has the person he loves. I feel like he sacrificed me and out relationship like it was nothing.
How can I move on and find some closure?
Firstly, I want to say that I'm really sorry this has happened to you. It's absolutely shitty and it's not a situation that you really should have been put in.
I wish that I could tell you that if you found more "seasoned" polyamorous people you would somehow be safer from this sort of outcome -- and that might be what a lot of other people would tell you. But in this case, you seemed to have done all you could do. You had regular check ins, you had a partner who claimed he didn't do hierarchy (and that may have been his full intention -- until it was put to the test) and there wasn't really anything you could have done differently to address that.
Unfortunately, sometimes people don't know they have a boundary until it's been crossed. For some people, polyamory isn't an orientation and it isn't something they necessarily feel they have much loyalty too -- and there isn't anything inherently bad about that. I don't think your partner is necessarily a bad person and I don't think that his partner is either. If I had given him advice, I probably would have told him to wait a bit longer before deciding to "de-escalate" your relationship. Breakups are usually painful and difficult and most people are going to want to find a quick way to stop the pain. Making a decision that rash doesn't seem like it's helpful but I wouldn't assume that he's necessarily fine and dandy. If he is prone to deciding things so suddenly, he could wind up feeling the same way about losing you.
The important thing to remember here is that incompatibilities are not necessarily your fault. It may help, even if you do want him in your life, to have a period of separation from him where you can work out your feelings and process them. There's a lot of understandable frustration and anger you can feel -- and rightly so. Even if he doesn't want to think of it in these terms, he has more or less sacrificed you to save his other relationship and done so in a very sudden way. While he was given this ultimatum quickly from his partner, he didn't necessarily have to act as quickly.
Allow yourself to feel and let some of this anger and frustration out of your system. Consider working with a polyamory friendly therapist if you feel like you need a more professional look in on the situation. Think about what types of conversations you want to have with future partners about this situation. I don't think you'd truly be able to completely prevent this type of situation from happening again, but getting a gauge on whether or not future partners have really thought introspectively about polyamory might help feel more secure with them as I'd expect this situation to cause you to feel a lot of worry in the future about your place in your partners' lives.
Also remember that a lot of people are in monogamous relationships for a very long time when their partners decide that they didn't actually want to live the life they are living. Sometimes people choose the wrong path in life or they end up growing and changing in a way that the path they've chosen no longer works for them. It's not something that anyone can fully prevent or predict. Sometimes when we stop trying to find a way to prevent things like this from happening we can also relieve ourselves of the burden of believing it happened because we did something wrong.
I hope this helps and good luck!