If we've had any kind of functional parenting in our childhood, we've been taught to be respectful of others, perhaps to even be deferential to people who are older than us (depending on your culture). Add to this, if you grew up in middle-America where being nice and non-confrontational is a societal norm, then you know how uneasy (maybe even downright frightening) it is to actually BE confrontational.
Abusive managers bank on this. It is this fear of what others might think, or what drama might ensue that keeps too many of us quiet. Even if we manage to find the words to respond when asked something inappropriate, or when boundaries have been crossed, if a bully can smell fear or intimidation, they often try to double-down until we are silenced.
The older I get, the less shits there are to give. When I was young I was easily intimidated. Now? Yeah, just try to cross me and you'll find out when pushed to far, I have no problem making my case clearly and without reservation. That didn't come overnight. It took practice.
There are people from all walks of life and backgrounds that find it hard to stand up for themselves. To do so might mean wading through a cesspool of insecurities. So many people have made inner vows that avoid this discomfort at all costs. I used to be this person.
What I've learned is that if you don't learn how to advocate for yourself, no one else is going to teach you. The world doesn't give a shit if you get screwed over.
Indulge me with a bit of advice: Try role playing with friends. Let them be that abusive obnoxious boss so you can get some practice time in saying what needs to be said. Pick opportunities to move from practice to real-life. Don't start with a major problem. Start with the little problems that give you an opportunity to speak up.
Before you know it, you'll have the confidence you need that telegraphs to your boss and all your co-workers that you are no doormat. In fact, someone would be well advised to think twice before crossing you.