I don't know you... to me, you are a person on the internet, with a life very different from mine, a life you describe and present here in a certain way.
But this is the feeling I get from your story (and I've read the whole blog, and Matt's thread, from the beginning):
to me it sounds like you never started with what seems to be the most important key to succesfull relationships, mono OR poly: communicate, communicate, communicate...
Those 12 years you've all been together seem so littered with assumptions, misunderstandings, not being honest with yourselves and the other players.. it is just so sad to read.
I don't have advice or an opinion. I just wanted to point out that maybe, poly isn't the problem here. The lack of honest communication between people who are in a relationship together, and the lack of true self examination, the willingness to look inside and see what is going on, is.
I wish you good luck...
early forties, straight.