This sounds very similar to my prevous experience with therapy. I had this stupid attachment to xT that for a long time I was too ashamed to admit even to myself. I never managed to get to the point where I could talk about it with her, and my attempts to steer the relationship in a direction where I maybe feel safe enough to do so ended in complete disaster. I somehow managed not to develop this sort of attachment with my new T. Meanwhile, she somehow manages to meet a lot of my needs way better than xT ever did. I have yet to make sense of al this, but I think part of what triggered that very painful transference with xT was that
1. while I was trying to focus on how this relationship was not like the one with my mother, she (and or the staff at the day hospital place we met) was pushing me to focus on the similarities. so instead of being allowed to shape the relationship at my own pace in a way that feels relatively safe-ish to me, I felt impotent and unheard, like in relationships with significant adults from my childhood. With current T it works more like a partnership than a parent-child thing, and yet I get the validation, acceptance and consideration of my actual needs that I was hoping to get from xT (still stings that I didn't get it back then, though).
2. she became unavailale - both in a practical sense, because one-on-on sessions stopped (because it was a group therapy place and I already had more individual sessions than it was the norm, but it still felt like rejection/punishment), and as a result of her (self-professed) avoidant tendencies, which to me came across as her not taking my fears and hurts seriously. Then that therapy ended and it was several momths before we started therapy again (individual therapy this time). Even then the avoidance thing remained a prolem. With current T, it's just her going on holidays every now and then. I guess I have a considerably easier time coping with those than you do, so it's quite possible that even the reality of a well-functioning therapeutic relationship is triggering for you?

What I'm trying to say with all this, that my sense is that some of the elements that trigger this painful transference might be the same elements that don't work well in the relationship? Precisely because our dysfunctional attachment figures from the past? Like, T feels a lot less motherly than xT did and yet fulfills the role of a mother far better. Some things that I long for are still missing, like the sense of closeness (that xT kind of 'promised' but it was fulfilled in painful ways more often than nurturing ones), but I'm starting to hope that might also be possible after a while.