I think it is mental to some degree. For example i can talk to myself alone in my room without much of a stutter but if i try speaking to new people or find myself in situations that i find intimidating i stutter soooo much more. (I gets EVEN worse if they ask me to repeat myself). THOUGH i also feel like there have been times when this shit gets physical. Sometimes when i speak i tense my facial muscles do much i get out of breath talking, literally making my stuttering automatically worse :/ Self love and confidence really does help you at the very least feel more comfortable in previously uncomfortable situations. Stay positive everyone!:)