How do you deal with customers insulting you for your stutter?

How do you deal with customers insulting you for your stutter? As the title suggest, this question is for workers that have a stutter. I get insulted by customers for my stutter all the time. They say I take too long to take their orders because of it. It really upsets me because I try really hard to suppress it, but when I get stressed out it comes out. How do you deal with people insulting you like that over something you can’t control? I haven’t talked to my manager about it because I felt so self conscious about it and felt so embarrassed.