Here's what I'm thinking about this kind of stuff as I did think a lot about this recently. Apparently some claim that you (general you) are supposed to be fully vulnerable in therapy, while the therapist is NOT vulnerable, sometimes not even transparent, congruent, let alone authentic. And the empathy of the therapist is finite, too. And it's a relationship where power inequalities can easily creep into it because of all that.

So I think that will make therapy harmful if you make yourself too vulnerable like that, no matter if the therapist has good intentions consciously. Good intentions or following some protocols on "boundaries" will not be enough on its own in such an inequal relationship.
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What would help with this fundamental problem in therapy is if there was mandatory supervision, and objective evaluation of progress and feedback provided to the therapist and then adjustments made based on that. Plus some better selection process to find a good match faster. Until then, such "deep" therapy is going to remain a mess. Risky and not worth it. IMO.

It could be also helpful for you to analyse what this deep thing inside of you it is that was triggered in that therapy. Maybe it was just the power imbalance in the relationship, maybe it was something else, maybe would help you with healing to identify and verbalise it (or at least have some images or or felt sense or some other content for it before verbalising the insight)

If therapy made you feel worse, then there you have your objective evaluation and feedback on how well it was working. It wasn't working. Of course, it's possible to be like, have two steps forward, one step back, but I don't think that's what you're talking about? And I don't think you should blame yourself about not making yourself even more vulnerable by talking about your childhood and so on.

If you sleep better now with the new therapist, that's another indication that the previous therapy wasn't helping you. Of course only you can decide if it was helping you, if you feel better, but that's just what it sounds like to me from what you've written.

I think a lot of therapists are NOT aware of what these feelings can be like. I don't think it's a real explanation or helpful to just chalk it all up to BPD, either.