My T is getting better at spotting when I'm starting to dissociate and will try to ground me before it goes too far. She asks me to tell her if I think I'm starting to slip away, but usually I can't manage it. If she notices she will ask me if I'm still here, try changing subjects to something lighthearted...sometimes switching topics quickly can pull me back, sometimes not. She lets me know it's ok to not talk, that we can just be there together, that I can lie down and sleep if I need to, is there anything I need from her etc. She would never stop the session, I think she's all about letting me know it's ok to be me, whatever state I'm currently in.

Would it help you to let your T know you're afraid of dissociating heavily before you bring the difficult subject up? So they can be prepared to help you in the way that you need? Hope your session goes well.