Have you heard about William Parry's "valsalva maneuver"? I highly recommend learning about that. As a stutterer myself, it quite accurately describes the exact method how stuttering happens. It's called a valsalva maneuver - a social situation triggers a psychological fear response, which then leads to an involuntary blocking of the air passage in your throat, which in turn creates this invisible wall you have to traverse to get your words out.