Man, newtoday, that hurts to read. That is a tough way to live poly.
I get one night a week, too, but there are opportunities for overlap time when the metamours are together. These are positive experiences and add to the individual relationships because we all get along well. There's an undercurrent of support for the whole structure which I didn't properly appreciate until I read your post.
One of my SO's requirements in a relationship is that he is allowed to talk positively about his girlfriends to his girlfriends. If someone doesn't want to hear nice things, that's a potential flag.
If I were in your position, I think I would have to check in with myself to be sure stability wasn't a need in that relationship. How can you know from one week to the next what you're going to get, from him or her? Even if he loves you with all his heart? How can you find trust in that scenario when she is determined to erode it? And the more she hinders communication, the easier it is to erode the trust.
In theory, another relationship could provide some stability for you, but it's not reasonable for me to tell someone to just go out there and find another great love. Sure, no problem! That process is working so well for me.
I'm not much help, but I'm rooting for you.