Yeah, our stutter changes depending on our mode of performance. Try this: Record yourself. Play it back to yourself. Then record yourself again - but this time, play it back for another person. A new recording for a new situation. Did you notice a difference in stutter? - At any rate, "mental" is used by people in slightly different ways. Yeah, for sure, there's a "mental" component to a stutter, but what exactly that amounts to will depend on the stutterer in question (and the situation too). If we're talking about mindset, that's something we can work on, such that our mood might improve in general. It's not nice being in a bad mood about some ailment or other, so it can be beneficial to get better a coping.