Cleo, I think I understand a little of your experience. I also often zone out when the conversation gets onto difficult stuff (csa) and I feel like I'm sort of floating, and that my legs are a long way away. I can't move and I can't speak, and I hate it.

I know you said you have to say everything, is that a rule your T made? The way for me to get things out has been to write them down, sometimes through email, sometimes I've written it out and brought it to session, sometimes written it (very badly) in session and T reads it.

The last way is helpful for me, I wonder if you can say to your T that you struggle to say some things because you feel that you zone out and it's scary, and could you write it out for her? For me, if T knows and can ask about what I've written, it's much less overwhelming than trying to find the words myself.
Good luck.