Well, you finally put your foot down, and asked him to make up his mind and choose.
That part was good. I am glad you did that, stopped the wishy-washy behavior on that point, at least.
When he gave his answer of "Not you, Greenjade" you wobbled. It does not matter if it is to go off to be alone to think, be alone permanently, or go off to be with her. How it affects you either way, it is "Not you (right now), Greenjade."
It sounds like that answer wigged you out a bit. I know that's hard to digest and accept. It was not the answer you hoped for.
But in having him hang around some more, be aware that you could be in the bargaining place with that choice. ("If he isn't picking her, I'd rather he be with me, until he decides whom to be with.")
Your suffering might not end there. You might find yourself wondering when he's going to bail on you next, since he's bailed once already.
On his end, he may grow resentful that he made a choice for himself and then reneged on it. He could direct that resentment at your head rather than at his own decision-making process and wishy-washy behavior.
Would a 3-6 month trial separation for both of you to collect your emotions again really be so bad? You could then make life-changing decisions from cooler heads, not all het up, whether the decision is to break apart, or do the work of repairing, so you can stay together.
You could benefit perhaps from knowing you can make it on your own -- that being alone isn't so bad.
He could benefit perhaps from knowing what it is like to miss you. When he says and does things that hurt you, and takes you for granted, because
"he knows our relationship works" -- that's not cool.
I am concerned that when you say this, you really are talking about
yourself:
I think he is scared to walk away from us, because he'll end up feeling like he is now, but with the women in different places.
I could be wrong, but I think that could be reframed as:
"I am scared for him to walk away from us, because I will end up feeling like I am now, but alone."
Is that where you are at emotionally now?
You mention
living in an isolated way since moving to new town, not socializing much, not having friends other than through the kids, no lifelong friends to unload on, and not able to talk to your mom.
You mention your willingness to do things you
"would rather do than lose him and be alone". I get that it is scary to think of a future that is different than the future you originally planned for your life. But remember,
you felt this too:
I think I might be done. I'm hurting too much. I'm thinking about telling him he can just have her. When I think about telling him, this calm/numbness washes over me.
In the alphabet soup of emotions you might be feeling day to day, or even hour to hour, you could go toward the choices that bring you calm feelings, not choices that bring you more worked-up crazy stuff. Feelings sometimes make no sense, but sometimes serve as a guide. You could listen to your feelings.
Asking him to stay from fear of being alone, while initially relieving because you are not yet alone, is still not him picking "Be with Greenjade" from the beginning.

That's him picking "Not Greenjade," and then "Letting myself be persuaded to stay."
I cannot know his intentions with choosing to leave and then staying, but it could be things like:
- He's a wishy-washy guy who cannot make decisions, and he's going to ping-pong before making a break.
- He wants to make it so you are the one to leave, so he can tell himself that he's really "a nice guy" that got a raw deal.
- He's enjoying all the drama/attention; feeling "wanted" strokes his ego.
- He's testing your "loyalty" to see which new kind of rudeness he can push for next, since you have accepted and tolerated this much so far.
- Or, something else I cannot see.
The "ugh factor" increases down my list, but even the mildest one, him being a wishy-washy decision maker, is still not "I pick Greenjade because I love her!"
You deserve to be with someone who JOYFULLY AND ENTHUSIASTICALLY wants to be with you, even if their
other relationships in the poly network are wobbling or ending, not someone who is lukewarm about it. You do not exist to be the back-up plan.
Later, some doubts could start creeping back in. "He bailed on me once already. When will he bail again?" The relief of the initial, "Yay, I'm not alone!" will wear off, and change to more worry, anxiety, suffering.
Please be careful with yourself. You could accept your short-term health is just full of suckage. Fine. So be it. Hard times right now. Focus on your long-term health later down the road. What are the best things to do preserve your long-term health?
Okay, so he's chosen "break up with both." Since on your end that involves a lot of legalities, I would encourage a trial separation, to make sure that's the correct life-changing choice. Him bein NRE-drunk could be taken into consideration. So try it on. Be separated, but not yet divorced.
As hard as it is, both of you would get a chance to look at this situation with cooler heads, so you both (especially you) can take a better assessment.
Don't be afraid to shine a light in dark corners. You've already done so much and come so far in putting your foot down and asking, so you can know where you stand. You know where you are at now. That's progress, at least, even if it is not yummy tasting right now, nor what you hoped for.
Continue to process. Just do the job in front of you, looking out for your own best long-term health (mental, emotional, physical, spiritual), with or without him. Maybe you want to book a counselor to help you sort it all out.
Hang in there.