I believe that you can work on attachment issues, and possibly get some resolution with a therapist... but I agree that it takes a really, really strong, centered, good therapist to help with the work.
I think it requires a therapist that has dealt with their own stuff successfully, has a stable sense of themselves, and can withstand whatever a client throws at them without reacting from an emotional place (i.e. being defensive, or angry, etc.)
I think these therapists are really, really rare. I've seen a lot of therapists, and there have been some really astonishing failures (I know I've mentioned it before, but there was one that I asked about phase oriented therapy, because the expert who diagnosed me with a dissociative disorder recommended it and I had never heard of it... this new therapist answered me angrily with, "I've been doing this longer than you've been alive!" - yikes!)
I don't have any kind of proof, because I still haven't found a therapist that is able to really help me, or create the kind of environment that I seem to need to feel safe and open up.
Have you checked out this blog:
https://boundaryninjatales.com/
I'm not sure how active it is these days, but it's got some really amazing information in it. I'm looking through some past posts, and it seems like the writer was in therapy (on and off) for 20+ years before she found her current, awesome, attachment-focused therapist. I *wish* I could see her therapist, seriously... he just sounds so.... amazing and competent and grounded.
I actually tried to describe some of that to my T, and he said something like "why are you telling me this?" Ugh.
Her T talks about how he is an "attachment figure", and how she should contact him any time she needs to, and he responds to her! The idea is that she has to learn to trust that he's there for her (and then he actually has to be there for her). I'm probably messing some of this up, but it seems to have worked really, really well for her... I think she's either finished up therapy or might be close? But if you read her writing, she's dealt with some really horrible childhood crap (and has a long history of therapy from before this T too!)- and this T seems to be helping.
I think she explains things well, but I can't pull up a specific entry right now to point you towards. It's worth a peek though, and you can use the word cloud on the right to find attachment-related articles.
Anyway, I'm really sorry to hear that things ended so badly with the last T. I thought that you had gotten a lot out of seeing her? No? Was the ending bad enough to erase the good things? Either way, I'm really sorry to hear it.
And, I'm surprised that she admitted her attachment style to you! I'd think that a T who works with clients with attachment struggles needs to know how to keep a secure attachment herself! Hmmm....
It's all really hard. I wish there were better answers. I wish that the really good Ts were easier to find!