I have never had any place I called home, probably because I moved so much as a kid. Which is odd, as nostalgia is such a strong force in my consciousness, especially my aesthetic consciousness. I suppose I had a series of homes, so I guess you can't technically call my odd fits of nostalgia "homesickness."
Yet as attached as I got to anywhere I lived or traveled to, I always felt like the feeling wasn't reciprocal, that my surroundings didn't love me back. So whenever I left, it was almost a relief, because in effect I was leaving a place that I never belonged to, to which I had only been a temporary disturbance. Maybe it was because I never stayed in once place long enough to learn to belong that I never learned to think of myself as having any more significance to the world around me than a "disturbance"
In fact, I was surprised to recently discover that some of my classmates had called me their "friend" in high school, because for the longest time, I had believed that they never liked me back, or considered me a nuisance. I had assumed that they had kept me company during extracurriculars, spare time, and during lunchtimes simply out of convenience. And this was even though I'd known some of them for 5 years or more. Even now, I have trouble making friends because some part of me firmly believes that no one will bother liking me back. Old habits die hard.
(I may not be entirely wrong about companionship arising out of convenience, or the ephemeral, tenuous nature of friendships. I was wrong, however, to doubt that such relationships can be genuine while they last, circumstantial and brief as they may be.)
All this to explain that I fear I don't understand homesickness very well. The closest I got, I think, was missing my mother and brother when I was separated from them. And from my ex-boyfriend as well, who was more of a surrogate parent to me (during a time of great strife in my own family) than a lover. They were the only constants in my life, for much of my life.
I got used to leaving so much that space, in my mind, is indistinguishable from time. I read once that some Greek philosopher (edit: I now know, thanks to my mad search skillz and Wikipedia, that said philosopher was Heraclitus) said that you cannot step into the same river twice, and the idea really resonated with my own experience of the world. Even now, when I think of the autumn leaves in Pennsylvania, the foggy hills of the Bay Area, or the blue skies of San Diego, I think of specific moments in my life and feel all the more wistful (a poignant pain, pleasurable in its own way) because I know that those sensations and those memories are no more: even if I went back to America, the land of my childhood, it will not be the same America that I left behind.