How do I talk back to someone without coming off as weak and pathetic?

I'm not so good with words when it comes to speaking up about myself. As a child, I was a victim of stammering which also made me an easy target for the bullies to pick on. And even as an adult, there have been times where I've been talked down or outspoken because I either couldn't talk fast enough, I fumbled on one of my words or I couldn't come up with something clever on the spot.

Does anyone have any suggestions on what I should do? I don't lack confidence if that's what anyone's wondering. But when it comes to someone starting beef with me or just picking fights and talking down on me, I don't usually know how to respond and it's really embarrassing and frustrating and makes me feel aggravated and worthless.

Any tips and suggestions? Please.
How do I talk back to someone without coming off as weak and pathetic?
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