Here's a thought, Hope... I feel like detachment is letting go of all expectations-- meaning, not just for reconciliation but for D. I don't necessarily think that in order to let go of your belief that this will end in R you have to hammer yourself over the head until you believe that you'll never get back together.
To me, the key is really understanding/accepting where you are RIGHT NOW, in the moment, which is-- as difficult and heartbreaking as it is to accept-- that your M as it was is no longer. That your H is simply not, at this moment, the H that you deserve. And regardless of what happens in the future, your primary responsibility in this incredibly difficult and confusing time is to be there for yourself and for your D4. If it is painful and upsetting to interact with him and get drawn into his circus... don't. Let the feelings flow over you and then out the door. And redirect all that amazing energy you have back into yourself and your D rather than wasting any on him.
You can count me in as one of the readers on this board who has thought and still thinks there is a chance for you two. But, right now... none of that matters. Whether it is meant to be or not between you two is not something you can control or need to worry about. For me, focusing on what I could control-- myself-- and letting go of what I can't-- his actions and thoughts-- has really helped me, though it is always a work in progress. In fact, typing this out to you reminds me that even though I'm frustrated by his unwillingness/unreadiness to talk about the A... his timeline is his and not for me to dictate. I can only control my own and if it ends up that his timeline stretches out beyond what is OK for me... then that is another decision I'll need to make, at that time. Understanding what it is that YOU need and supporting those parts of you, whether we call it GALing or whatever-- is the most important thing right now.
Another thing that helped (and continues to help) me is that letting go of everything when I'm with the kids. I had so much fear about S and D and what that would do to them, and I can relate so much to all that you are saying about being with your children. But I was carrying that fear with me when I was with them and it was was interfering with me just enjoying them. I worked on that quite a bit, to be really present with them, and I also think that helped me in the overall process of detaching from my fears and expectations around the relationship with my H.
Me (46) H (42) M:14 T:18, D9 & D11 4/19 - 12/19: series of escalating BDs 9/20 - present: R and piecing