I also realize that no two people or situations are alike, and that I have no idea what you're going through!
But from a fellow introvert who has had success with interviews: an interview is a good example of a social situation that's fairly "hackable."
By hackable I mean an interview is something where, at least compared to an open-ended conversation with a stranger, there are certain... branching paths of conversation?... that you can practice ahead of time.
The inevitable questions about why you're leaving your current job are definitely a thing you can practice. I don't think it's necessarily bad to mention some negatives regarding your current position and why you want to get out of it. If it didn't suck in some way, why would you be looking for a new job?
I think a good answer to the "why are you looking to leave your current position?" question would involve:
1. What you like about the new company ("You guys seem to hire really good coders, you're growing as a company, and I like your approach to A, B, and C.")
2. What you want to achieve at the new company (ex: "I'm looking for an opportunity to do less support and more coding.")
3. Optionally, something about what's bad at your current company. Preferably in a positive context. (ex: "I'm really excited about building modern web applications, and I'm not able to achieve that at Company XYZ because we've committed to supporting IE6.0 until 2050 because it's what the owner's mom uses.")
Anyway, it's not easy, but it's.... hackable. Unless it's a really weird interview, most of the ground you'll cover is rehearseable. Good luck!!