> I have a hell of a time explaining this to my more extraverted 
  > friends -- many of whom "don't see it," because I'm not 
  > particularly shy or reserved in social settings. [...] 
  > because people don't "buy" my introversion. They just think 
  > I'm avoiding them.)

God, yes. Huge sad ongoing thing in my life. This is a constant thing I do. I'm not awkward, so I make these friendships, and then I unintentionally hurt people with my introverted tendencies.

I feel you.