fight or flight response

I have been doing the work to clean up my thoughts about my mother in law so that I can feel better and have a better relationship with her. However, when I am around my mother in law (even talking to her on the phone), I have a physiological fight or flight response. My heart rate quickens, I get flushed and flustered, and I am on high alert. Sometimes, my fingers even go numb. As much as I tell myself new thoughts, it’s as if my body is hanging on to the trauma that I associate with being around my mother in law. My action is that I then shut down (either leave or disassociate and pretend like I am not there). What do I do?