Bookbug - I'm so sorry for what you are going through. As I read your story I felt a real connection to you because of my experiences over the last year. I was part of a MFFM quad which was powerfully passionate and eventually exploded brilliantly due to inherent personality clashes. We each contributed our own issues to the mix, and my husband and I discovered some marital issues of our own but one of the most significant problems with the quad was the terrible quality of the other couple's marriage.
When we first became involved Star and Carter* gave the impression of a stable open marriage, and were very much convinced themselves even that they were in a healthy relationship. Over the next months though we began to discover the cracks in their foundation, each of which my husband Jute and I tried to rationalize away because we deeply loved them by then and wanted the poly family of our dreams so badly.
I'll avoid the sordid details, but long story short they are divorcing now, and Jute and I were separated for a few weeks, but are now together and happy again. Star and Carter get along much better as friends than spouses. Jute is still trying to be friends with them and is still mourning the relationship I think. I can be friendly with them but we are not close friends. Personally I finally decided the drama was too unhealthy for me to be close to them, even though I miss them very much. I decided I needed to move on.
I would be there for them in a heartbeat if they needed help. I know what you mean about not being unable to 'un love' another. But I also had to decide to take care of myself. I couldn't wait (potentially a lifetime) to see if either of them could become healthy for me to have a relationship with. I couldn't put myself on hold while they were figuring out their issues, and trying to do so was literally causing me to question my sanity.
In my opinion these people need to become your distant friends for your own good. Focus on yourself, and what you've learned from this experience for a while. Throw yourself into a new project or activity. Meet new people, but you don't need to date others if you're not ready to do so. It took me a while to even feel ready to be with my husband again. About 6 months from their exit I have just now began developing a relationship with someone I've known a long time prior. Dating someone brand new at this point is probably beyond my emotional capacity.
Good luck and Blessed Be
*all names changed for the sake of everyone's privacy