I actually practice out loud, sometimes in front of a mirror just to practice my mannerisms. It doesn't tend to work very well. At the time of speaking, you just go completely blank. Preparation doesn't help when the only thing you're mentally capable of doing is being ultra-aware of every inch of your body and facial patterns.

I have bipolar and social anxiety. When I'm in my "up", I can speak in front of a large crowd fairly easily and can't even come close when I'm in my down. Thing is, the two comorbids work together. If, for example, I mis-say something, my mood has a tendency to completely drop, which affects my social anxiety, which affects my mood, etc. It tends to last for about a week after that.

Unfortunately, this sort of thing can't be labeled as introversion vs extroversion. The problem is very, very complex and has many facets. Giving it an introversion label doesn't do any justice to the intensity of the problem. If anything, canonicalization of this sort seems to do more damage than good, as it gives a false sense of understanding of mental health.