I am sorry you are hurting.
You write well.
So let me just point this out. This dalliance with this woman? Shoosh it aside for a minute. Look at what you write before
airing out the story about the woman and how you felt before this even all got started.
I had realized we had grown apart and I would either have to leave him, causing major upheaval in our lives, or we would need to seek professional help. My mistake, I acknowledge, was running from that realization, smack dab into the poly conversation, and entertaining the notion that, maybe if I could date other men and women, it'd be easier to live with him as best friends.
Basically you could ask him if he's up for
a) Close back down in order to see a counselor and get back into right relationship that way
b) Go see a mediator/lawyer to negotiate a clean, fast split and get back into right relationship that way.
That's about it here. The woman at the beach? That's just frosting. Your cupcake was already in progress by the sound of it.
But don't do another big circle around the bush running from realizations and avoiding what is in front of you. If YOU do not feel like counseling is worth the bother or you cannot be fully present and vested in that making it work thing? Don't even put it on the table. It's basically a split.