I think many of us have been in similar positions. Not fun. My organization was agist. They kept getting rid of people of a certain age and salary bracket, and replacing them with younger and cheaper.
I don't have advice. I think it's a tough situation you are in.
I will say that things change in the workplace. People move on, alliances change. Sometimes things get better.
My sib worked in a heartbreaking job where he saw the worst of people and his department got blamed whenever there was a domestic incident.
His ability to leave it behind when he left for the day was always amazing to me.
Maybe, as well as assertiveness, you can practice compartmentalization.
You know, leave the work in the rear view mirror when you are not there.
Tough, I know. Good luck.