I was surprised that my original post elicited so many replies. I suppose it struck a chord with many people, and in a way that's reassuring: it's nice to know (at least now, after so many years) that your own experience wasn't unique and that there were many, many other people all going through much the same thing.
I don't know whether many of you felt my separateness and isolation to the same degree. Maybe you did, or worse. It was a bit strange. I wasn't ever lonely, and I was very well cared for and protected by my parents, as was my brother, but I felt inwardly as alone as if I were marooned on some desert island. And as self-sufficient. Mind you, I think that suited me more than I would care to admit. Meaning that you're supposed to be gregarious and people-loving, and socially well-adjusted, and not be a secretive soul or some kind of hermit. But I'm unrepentant about that. I was the person I was. The modern me is finally learning how to be different, and I think that something that was long dormant is finally having expression, because I do now seek out people and enjoy their company. That said, I will always want freedom and space; but I am getting intense satisfaction from my new outgoing life as Lucy. And believe me, actually meeting my online friends is an unalloyed pleasure to me.