TerpGal, don't be so hard on yourself. If being assertive is new to you then it will take time to change.
I'll share that my very first relationship years ago was pretty bad. He was verbally abusive and beat me down to feel like a doormat. When we finally split it took me a very long time to come out of my shell. I had a good support system as I hope you do as well. They really helped to pull me out of the darkness.
Now, as far as being the b@t h at work. You bet I will! I work in a international company, many nationalities. Some of our gents don't stop to think before they lash out. For a while I sat quietly, I need my paycheck, LOL. Seriously though I grew tired of their abuse and when I became pregnant with my second child those pregnancy hormones kicked in! I stood up for myself, I don't always have to be a b@t h but I will if necessary. There is a way to say no and just stand your ground and if someone does not like it then that is THEIR problem.
Take baby steps, keep saying no when you have to and before long it won't be so hard to tell them no.
Unfortunately I am not full of wisdom but I hope this helps a bit!
Take care!!!