Hi Josh,
I hope you don't mind, I read you post and join, because I am experiencing something very similar to what you have described with Jack. I thought I would share something that is helping me. Having acknowledge a much younger (and untrusting/fearful/despondent) part, I also feel guilt for having left her so long but- that's part of the nature of DID and somewhat unavoidable at times. Intention means a LOT, your intention wasn't to course further pain to Jack and the fact that you are willing now will mean more than you may currently realize to him.
Heres what helped me; Trust can't be assumed, so rather than trying to understand, figure out and 'know' what has happened to her, with the urgency she has approached me with, I'm just trying to gain some trust. This also helps me as I don't think I'm ready to hear what she might have to share, but I don't want to ignore her anymore. So instead I just sit with her, a lot, and make it as consistent as possible, lets us get familiar with each other. i figure in doing this she may come to trust I'm not going anywhere, I'm with her for the long run and there is no rush (dispute the sense of urgency the emotions evoke).
Maybe it would help you to? Just to sit with him, in what place he feels safe (i'm presuming that might still be an internal place?) ofter reassurance of the present, be the "adult" in the room thats in control he can learn to trust. I can't stress how helpful it has been for me to "pace" and respect the time it takes to build trust first. This also helps when heavy 'stuff' memories, emotions of younger ones surface, if there is no trust in each other, then internal chaos seems to prevail. Having said this I struggle with it, massively, its not easy but i think it can turn down the chaos. I'm also encouraged to see the support you guys offer each other and hope my joining doesn't make anyone uncomfortable (please say if so).