GC -- you have my serious sympathies.
This manifested in a big way for me with current T around time stuff (going over with the previous client and stopping right on time when it came to me).
I don't know if this will help but some of what I've done has been around noticing when it comes with other people -- you're definitely not as crazy as me but yesterday for example, I was waiting to get a massage (from a fuzz-faced, hipster-ish 20-something dude who I was seeing for literally the first time -- my interest in him couldn't possibly have been lesser than it was).
I heard him finishing up with the previous client (payment stuff with small chit-chat) and I was SO.FRICKIN'.MAD. It made no sense whatsoever but the burst of jealousy I felt about the really familiar way they were talking to each other drove me nuts.
Another time I distinctly remember noticing this -- when I'd literally been hanging out with someone at a meditation retreat for a few minutes and I noticed her talking to someone else with some kindness and boom.....I was fit to be tied.
Until then, I'd not given her any more thought than thinking that she's a kind, fun person and that's about it.
For me, the only thing -- and I need to do a whole lot more work around it, I know -- that I can currently trace it to is somehow feeling like I'm just "not enough" in any way, to hold my mother's attention (I had my father's attention in screwed-up ways).
There's a lot of stuff mixed up around it -- I never knew what would be of sufficiently high level to keep my mother's sustained attention (it's a feat I still haven't managed to accomplish!).
So, in seeing current T or someone else with other people, I assume they're either still thinking about the previous person or going on to the next person i.e., I've been bypassed altogether.
I'm not sure how to get to that point where it feels like I have enough to hold someone's attention in full and actually feel like it's real and authentic -- I feel it at times (in therapy and elsewhere) but it's rather elusive.
I guess the only thing I'll say is that whatever it is, it has to come from within me.
But yeah, I have no idea how I'd do if I were to try sorting that out while also dealing with my family of origin -- hang in there and get out as soon as you can is all I'd say....