I wrote about my own experiences with introversion here:
Some thoughts on introversion
Right now I am at a point where I still prefer to be alone most of the time, but no longer find social interaction difficult or painful; the comfort level is pretty much the same whether I'm with people or not.
And I haven't really become more extroverted -- that is, I didn't get this result by imitating or trying to cultivate extroverted behavior. The interaction I experience now is of much higher quality than the stereotypical extrovert way of communicating.