Yeah, CBT sounds kind of backwards to me. Or maybe not backwards, just ignoring half of the picture. Like, yes, changing thoughts can change emotions. But where do thoughts come from in the first place? I mean, CBT-like techniques helped me in the past, though I never had official CBT therapy. Maybe with a highly trained therapist it'd have helped more. Anyhow, a lot of those "replacement thoughts" I still don't actually believe. I can remind myself that what I believe is technically likely to be not true, and it's somewhat helpful, but it's ineffective beyond a certain depth. I think it's fair to say that these techniques just don't address your emotions at the level you need. I have even less experience with DBT. But sounds like it should be applicable in your situation?

Re: your thoughts about L. You're probably right that she gets something out of it herself. You're probably right to worry that her new daughter might change what she gets out of it and it might affect your relationship. And that'd suck, and would be unfair.

I wish your T were willing and able to help you untangle yourself for L somehow. And that L herself tried to help you with that instead of working to maintain this very deep involvement. It sounds impressive how good she is at accommodating your needs (less so than in the past, but still), and it's heartwarming that she cares so much, BUT clearly all of that still isn't good enough and realistically, it can't be a therapeutic goal to maintain this level of enmeshment indefinitely. The relationship itself yes, the caring and closeness yes, but whatever has been going on between you lately, just screams danger to me. So I guess I'm saying that I feel your fear is actually justified. Probably not the end-of-world scenario it likely feels for your needy parts, but a very real danger of getting (more) hurt.