I struggle with this in therapy, a lot. Pretty much every time we talk about anything difficult, which is pretty much every session nowadays.

I know that in the beginning, my T wasn't aware that I was dissociating. I wasn't even aware of it myself. I've come a long way in recognizing it, and so has my T. I think, like you mobius, I was really good at hiding it and being able to continue with the conversation even if my mind wasn't all present. My T has learned to see the really subtle signs that I'm dissociating, and I've learned to tell her how I feel.

Sometimes I say "I'm not in my body" or "I can't feel my body" A lot of times my legs go numb, and I tell her that. But usually I'm fighting, really hard, to no just completely dissociate, and I'm counting on her to see that and to help me fight it. She tells me to open my eyes and keep breathing, since both closing my eyes and holding my breath seem to be ways I can push away the feelings in the moment. She gives me silly putty or something else to manipulate in my hands, or has me describe something in the room to her.