Wow, Nicole, I resonate with you with absolutely everything, from growing up in an environment where I was never allowed to say no to staying in a toxic relationship for years.
I try so hard to do it differently with my child and one of the phrases that make me feel more empowered involves a few extra words to the simple, full sentence that "No." can be.
I moved up to "No, I won't let you do this to me." or "No, I won't let you talk to me like this."
I truly hope my child will pick up on the message that it's important to signal inappropriate behavior and let others know you're not going to take it.