You know what? I could have written this myself.
I didn't learn this until several years after I became acquainted with MBTI, but introversion and extraversion don't correlate reliably with sociability. Real world levels of extraversion depend on both previous experience AND preference, whereas MBTI is supposed to reflect only your preference. For example, an ENFP who is repeatedly shunned or treated badly because she expresses her feelings will become more guarded with her feelings over time. It's not what she prefers to do, but it's necessary for her survival. This is what happened to me, and perhaps it's what happened to you.
When you're determining your MBTI preference, you need to rely on the way you prefer to behave when you're in your healthiest, happiest state of mind. I first realized I might be an extravert when I came home one day from an interesting dinner party. I came home so energized and raring to go.
That was the beginning of the chain of events that led me to conclude that I am an ENFP. (Well, there were several other clues along the way too. I kept noticing that I was the most assertive of the INFPs. When my feelings get hurt, my first response is to explain my opinion and engage the person I'm arguing with, not to shut down and feel badly. It's only when NO ONE around seems to understand me that I shut down and feel badly.) The bottom line for me is that it's not just enough for me to FEEL strongly about something. I have to talk to everybody I know ENDLESSLY about everything I feel strongly about. I want people to know who I am and what I stand for.