(Mostly just FYI) I left my former T because it was a rather bad fit in other ways -- yeah, I guess you could say she was street-smart or just really pragmatic.

I think for me it's incredibly frustrating mostly because I don't know how to explain certain things to her without having her somehow regress into stances that I find rather painful to watch. So, when she's not sure of something that I'm talking about, she'll default to a general validating / reassuring stance -- that sort of stuff irritates me by itself and worse still when it's clear that we aren't able to understand each other very well.

You're right about that feeling of distance and the sort of childhood experience that it evokes -- and yes, similar to what you described, it can yet again bring up a feeling of not being protected because someone who is supposed to do that is way too weak themselves, really.

For me, it also brings up a rather familiar (and painful) sense of not-belonging (in a very deeply interpersonal sense i.e., there is no 'real' [I put that in quotes because it's therapy but I've also felt that in other relationships] me-and-her or that someone like me could never find a 'real' place in her emotional world, if that makes sense). And, again, somehow that relates to the idea that this person is unable to make space for me in a real sense in their emotional world -- and acknowledge all of me -- because they don't have much knowledge or control over things.

The usual way I deal with it is that I don't talk to her about any of it if I don't think I can handle what I think her response will be. But, then it also becomes a case of her not knowing a whole lot about me (which becomes weirdly unidimensional). I then tend to correct for that and try to talk about it in ways that she could relate to / make sense of but in doing that, I'm really aware that I'm sorta kinda pandering and it just leaves me feeling worse.

So yeah, I'm not sure I have an answer -- all I'm able to do is bring stuff up when I'm okay with any ensuing frustration.

ETA: I wouldn't take the responses on the poll as any indication of popularity (as stopdog can tell you!). I believe it's largely a function of who's logged on and their interests etc -- it can be fairly idiosyncratic.