I have struggled with my T to let him know when the cascade is happening as well. I would keep in mind that it's not like your T expects you to suddenly catch it before it happens--that's expecting too much, in my mind. But if you focus on letting her know as soon as you are able to, it may get easier in time to notice when it's happening and let her in on it.

It got easier for me but it took a lot of time and a LOT of practice and misses where I couldn't tell T what was going on. And in time the dissociation became less profound and distressing to me.