To add on what I said, in the past I would have periods of great confidence in myself, even socially, I would just 'fly like a butterfly'. But that was more something like a enthusiasm, a emotional and psychological mood, that was never long lasting. After a certain period, I would turn to my "usual self", or even fall in a somehow depressive state.
It was a small trick I used often to make a good impression in certain situations when I felt i need to do it. That's how I got a job in a bank, until they kicked me out in two weeks when they figured out I wan't like that all the time..
After I realised that, I tried to make myself more stable, which is very hard. Because if I want to be a sociable aperson, I have to be like that ALL the time, not just when I feel good or when it comes or goes for me. I was in many situations where I would see someone I just met at a meeting a couple of days behind, and I just couldn't talk with the person, and just ignored him. And this can sometimes become really embarrasing and unpredictable.
So I think stability and a good balance is the key. This can be done with some discipline and some seriousness. Following your moods, checking them and trying to "adjust" the moods, to find the "mean", like Aristotle said. We INFJs have some serious moodiness (most of INFJs I know), with abrupt changes of disposition...sometimes very high, sometimes very low. And that's not good, and probably it never grows to anything, maybe just the degree of falls and ups, but if one wants to grow socially and in many other areas, I think stability is the key. Until now, it works for me. I work in a place where I couldn't believe I could work...customer service...that's quite something for a guy like me. I don't like the job (the job in itself is cool, but the idea of working a long time in such a place is not for me), I just want to prove myself that I can do it.
So its kind of like a rythm, and I have to keep that rythm, to work on it, to improve it, to stick to it, small and sure steps, until it becomes my second nature, like it is for many people, especialy for the so called "extroverts".