Yes, I think I understand what you're saying. I can definitely relate to this. Earlier in my life (before I deteriorated further) I could sometimes manage to "put up a good front" when first meeting someone. This wasn't always intentional, and it definitely was not scripted - it was just a surge of "energy" I would get as I did my best to compensate for my social anxiety. This mostly only worked in settings that weren't too informally social though - like a job interview, or the first day(s) on the job. I could "perform" in those types of interactive settings - at least at first.
With "normal" people - familiarity (repeated exposures to a person, or a group of people) increases their comfort-level - but with me, it often does the opposite. I have found that the more that I'm around a person, the more internal pressure I feel to be social - which has a paralyzing effect. As time with a person (or a group of people) goes on - I feel more and more exposed as an abnormally quiet, anxious, withdrawn, socially-maladroit person - which exponentially increases my feelings of shame and unease around people. It's bad enough that I am this way - and it's even worse when people are able to "see" it. The high level of shame and self-consciousness I have simply cripple my already-weak relations with people - for I'm always worrying about what they think of my quietness and awkwardness.
And often, getting off to a good start - or leaving a strong first impression with a person - can make things worse. When this has happened, I've felt enormous pressure to keep things going - but the "real me" puts in a stubborn bid to make an appearance. I don't know what it is about first meetings that can temporarily obscure some of the worst symptoms of my anxiety.
For example - I remember when I was in the eighth grade - and I went in for an interview for this private school that I was applying to. From my interview, I bet that they had scarce idea that they would be admitting a student with such severe, crippling social anxiety. During the interview, I didn't feel overcome with social anxiety - they may have thought that I was a pretty poised, "mature" person. Well it would all start to go downhill about 8 months later - when I was an actual student at this school. Somehow, the day-to-day social strains that I encountered at this school weren't nearly as easily-handled as the interview! The feelings of extreme self-consciousness and shame that I discussed above set in. The more the school-year progressed, the worse it got.