We think of nostalgia as a state, like reverie. But it should be capitalised, a proper name of a place, Nostalgia. It is created in our own minds, out of scraps of memory - events which were once real - so it is not a fictitious place. But neither is it entirely real.
We go to work and imagine home, and when we get home, it looks greatly like the place we imagined/remembered. At home, we remember work, or church, or a friend's home, and when we get to those places, they look much as we remembered/imagined. This tricks us into believing that other imaginings in memory really exist as well.
Yet Nostalgia is not quite a real place, not only because we can't go there, but also because the memories are unreliable. Recent brain research suggests that when we recall an event from 2005, we are not recalling 2005, but retrieving the last time we remembered the event, which may have been in 2009 or even only last week. There is considerable debate now whether a long-term memory is generally stable over time, or whether each time it is recalled makes it vulnerable to being changed. To make an analogy, a long-term memory is a solid object in the brain. But when we heat it up to reuse it, does it hold its shape and go back the way it was, or does the heating up make it moldable, changeable, flexible?
It is not a fully either-or debate. Even the neuroscientists who hold a strong view of memory persistence accept that subtle changes occur. And those who believe in extreme plasticity of memory note that some things do not seem to change, or not easily. The research seems to be pointing toweard more plasticity at present. When we heat up the memory, associated memories from earlier or especially later times seem to influence it.
Note: This may prove useful in treating traumatic memories, BTW. Chemically interfering with the memory-making process when a subject recalls the traumatic event seems to weaken the power, the emotional charge that the memory has. The events are recalled, but later bits of information - such as the knowledge that the abusive relationship is over, or we are not in a war zone anymore - get a chance to exert more influence on the memory. I do worry what the abuses of this memory-changing technique might be, however.
We have nostalgia for times and places that are now cut off from the whole. I do, at least. I have nostalgia for childhood places and friends. They connect to my current life only insofar as they affect me. I don't have much nostalgia for my children's early years. Those have a continuity with the present - later memories inform the recall of those earlier events. I have some nostalgia for places Tracy and I used to live, or for friends we no longer see. But any person or place that has maintained continuity doesn't live in Nostalgia. Not mine, anyway. When I go there, they are not residents - they are visitors like me if they are there at all.
Thus, I have some nostalgia for Romania and Budapest, which are in my recent history, but little for Chris and J-A ten years ago, and less still for my two older shildren ten years before that. They occupy space in the present. All memory of them then is but a piece of now.
Nostalgia is rather an island, then. Or a place accessible only by ghost trains or overgrown trails - rather like the traveling through the crack in time in WP Kinsella's The Iowa Baseball Confederacy. (Recommended, BTW.)