Hello,
Hopefully your therapist can help you understand when this is happening. Everyone experiences it differently, but sometimes it feels for me like I've gone completely numb. I also start to have tunnel vision (especially in therapy!), and those concrete signs can be really helpful for realizing what's going on.
Once you realize it's happening, grounding can be really helpful for bringing yourself back. That may look like observing your surroundings. Try to list aloud or in your head things you can see, hear, feel, etc. It also helps to become aware of your own body, focus on your breathing, and what you're thinking.
I hope this helps! Good luck with your therapy