Also, you take all that stress and put it in your speech mechanism which leads to poor breathing, tensed up vocal chords, facial muscles, jaw and lip. As a result of doing this for many years, your speech mechanism gets overly sensitive leading to stuttering at even almost non-tense situations which is probably why stutters get confused as to why you're stuttering rven when you're confident. I recommend seeing a speech therapist and practising in front of a mirror, prolonging the words to 3-4 seconds atleast, and reading a lot. I have seen massive improvement in mine. Its 70% gone, though the relapses are particulary bad, the improvement is massive over the past year. I've been seeing a therapist for 10 months now!