Yeah confidence issues can block flow of speech. But it's also important to know how speech occurs which is naturally. NOT by force or techniques when u consciously move your tounge in taught ways to manufacture words. Majority of stutterers don't know that speech happens in a natural way so they implement learned behaviors that aren't natural and they also start to distrust their speech completely and think they also have to construct sentences or preplan senteneces so they have ready responses when talking to someone. But for a lot of other people they have problems with confidence issues which can cause mental blocks or stress etc which effects can and does effect speech many times.