I don’t know how to stand up for myself

Ironically, ever since I was little, I’ve been through so many counts of people who’ve dominated, walked all over, or taken advantage of me. I’m 22 now, and even though I’ve been walked all over most of my life, I don’t know how to stand up for myself or defend myself. I don’t know if it’s something you learn along the way, but I never did. Instead I just cower, get overly anxious, think of the worst possible scenario, get depressed and at times have suicidal thoughts. At the moment at my work, I have a coworker who hates me, is out to get me, thinks I’m deviant and mean, harasses me in front the clients, and basically thinks she has authority over me. This woman is 57 years old. I probably sound really pathetic, and I haven’t gone into detail about my upbringing and the conditions I was raised in, but they still affect me to this day