I know how you feel about shutting down and not being able to speak. I get that too, though it's less now that I have built more trust with my therapist and have learned coping skills and have become more aware of my feelings. I think it's also part of dissociating.
Tell your therapist that you can't find the words to communicate sometimes, and that you need him to help guide you through at times when you feel shut down. The first time I was aware that I dissociated or had a flashback was actually in my therapist's office. He wasn't aware of it at the time, but I later emailed him and told him what I experienced.
That is when he realized I have PTSD. At my next session, we talked about what happened and he told me he would be looking for it to happen again and would be sure to ask me if he thought I was shutting down. In this, we have both become more aware of it, and we have worked through ways of getting through it in a way that allows me to figure out a way to communicate.
What is happening is actually normal-if you are triggered, your brain starts to function differently so that the parts of your brain responsible for speech are basically disconnected in a sense, making it hard or impossible to speak.
Bring this stuff up to your T, and definitely write down how you feel, or write down questions to ask, such as, "how can you help me find words when I don't have any or feel unable to speak?" Maybe that will help.