Gah. How Do I Break The Fear Of Speaking?

I m an adult. I can reason and logic like an adult. When my boss gives me more work than I can handle in a time scale, than I can reason my response should be to manage their expectations by saying "Hey you ve just given me x to do. I was already doing a, b and c and will try to get to x afterwards. Unless you want me to give this more priority?" And they know I have other things to do and somethings got to wait. But I can say that in my head, and I can t say it out loud. Its difficult to... Gah. How Do I Break The Fear Of Speaking?
https://www.myptsd.com/c/threads/gah-how-do-i-break-the-fear-of-speaking.52976/