Dorsal Vagal Nervous System
The dorsal vagal system is one part of our nervous system.
This system responds to life threatening cues, by causing the entire system to shut down, numb out, and disconnect from others. And if you suffer from PTSD you may be perceiving danger that is not in the here and now, but from a time there-and-then when there was danger.
Dissociation and the freeze or collapse response are all dorsal vagal responses.
This system was originally theorized by Stephen Porges in his Polyvagal Theory. And he coined the term neuroception to describe how our nervous system scans for subtle signs of threat or danger and makes decisions based on those perceptions, without ever involving our conscious thinking brain.
So, it is not your fault if you react to situations in a manner that seems irrational at times, this is the influence of your past trauma on the current functioning of the dorsal vagal system that sends you into shut down.
Part of trauma therapy is learning to recognize when you get dissociated, numb or frozen to both learn how to get yourself out of that state and to begin to learn what gets you triggered into that state to begin with.
Some ways of bringing yourself out of this type of shut down is to turn to a loving other (human or animal), focus on the sound of their calming, loving voice, petting your animal and noticing the softness of their fur can help engage your social engagement system.
Using grounding skills to help bring you fully back into the here and now can be good, noticing things in your actual immediate environment (sights, sounds, smells, etc.) can help bring you fully back into the present.