I feel so stressed when I talk that I just start mumbling random thoughts

I feel so stressed when I talk that I just start mumbling random thoughts I’ve been stuttering for my entire life (17 years). My stutter is a lot more controlled when I talk with my parents, but when I participate during class the stress gets over my head and I just end up repeating the first words about ten times until I can actually collect myself and say what I want to say. The most annoying this is that I feel like my stutter is affecting my grades cuz I have lower participation grades. I kinda want some advice and comforting cuz this pisses me off a lot.