Hi, I hope things have gotten better for you. Your description of yourself sounds just like me. I had a bully boss this passed year for an 8 month internship. I just sucked it up bc it was an internship but it was a really difficult time. I did some research on dealing with bullies and one thing that I found worked to some degree was I made an extra effort to befriend and go out of my way for everyone else. I aligned myself with others around me and this increased my sense of power by how well liked I was by others and how much others appreciated me. I actually saw a turn around in how this mean boss treated me. She had acted exactly as you described yours. I don't get why people need to treat others badly - they should deal with their personal problems in a healthy way and not mistreat others. But that's not the real world unfortunately.
Even my husband mistreats me at times bc I am very kind, not a doormat similar to you, but it can be exhausting and unfair.