I don't how to fight back. Please advice.

I was a fat kid when growing up and I have lost my ability to fight back. I don't know how to respond back to insults because it doesn't affect me, but I seem weak around my peers when I just let it go.

I know deep down inside I should respond but am afraid of confrontation. I want to stand up for myself but I don't know how to respond to an insult with a good "comeback" or overcome my anxiety when I do. Also with the SJW movement, is terrifying speaking back to a woman. I like any advice or suggestion on how to get my balls back. How do I regain respect w/o being a totally hated asshole?