Get yourself enrolled in some martial arts, Muay Thai, boxing. Something effective with a strong community. After a couple years you'll feel more confident which may help your stutter but more importantly words and conversations at work etc won't bother you as much when you know what you're capable of. In terms of standing up for yourself you won't need to verbally argue and give them the satisfaction, if they wanna deal with the situation for real invite them to your gym, otherwise they can stfu 🤷‍♂️ I'm sure I could have articulated this better but that's the gist of it, who wants to go back and forth arguing with people anyway.