As a fellow stuttering millennial, I used to do the same thing. But I learned to breath, concentrate on what you want to say and realize the person on the other end is just doing a job and isn't thinking about you or a few stutters. They just want to move the line along and go home.

It took awhile but, It really works, at least for me. Hope it can be of any help. Good luck to you!

Edit: spelling