When I was five, friends of my parents came over to visit one evening. After some time, they realized they couldn't find me. They finally located me wedged in the backroom between the dryer and the wall.
I've had long periods of time where I would take a different hallway/route if I heard someone coming, because it was too stressful to meet their gaze or wonder what I'd have to say if they said "hi" or smiled at me.
I also know what it's like to start driving to some place with lots of people or just new people... and if there's no one there I'd be letting down by not showing up, I'd finally break down when I got there, circle around the place wondering what to do, and then go somewhere else by myself and just never show, because of the terrible stress involved.
I'm a lot better than I was, now, thank goodness... although i still feel the anxiety sometimes even to feel sick to my stomach. But I've done it enough to deal with it and trust myself AND others.
I don't think that introversion = shyness. One can be introverted and not necessarily shy. I think shyness gets mistaken for introversion. Introversion is more just a preference of how to use one's energy (which direction energy gets focused in), what recharges one, etc.
Shyness usually involves a lot of fear and also doubt in one's own abilities to cope and a lack of trust in the intent of others... and maybe the desire to avoid self-embarrassment. If I enter this new and strange place without knowing the rules, I might make a mistake; if I go here, people might ignore me and leave me feeling invisible and worthless; if I go here, I'll have to "perform" and I feel totally inept and people will think I'm stupid or laugh at me. (And so on.) So there's a lot of negative self-perception involved with shyness, or at least a lot of fear that is not necessarily the same as introversion.