Nostalgia

From The American Heritage Dictionary (quoted from dictionary.com):

1. A bittersweet longing for things, persons, or situations of the past.

From Dictionary.com:

1. a wistful desire to return in thought or in fact to a former time in one's life, to one's home or homeland, or to one's family and friends; a sentimental yearning for the happiness of a former place or time: a nostalgia for his college days.

The past is no less real than the present; it is a real time with real things, persons, and situations. But the important thing to remember about the past is that you can't get there from here. And the things, persons, and situations you can get to from here are quite different from the ones remembered so fondly. One cannot return "in fact" to what one is longing for — it is simply impossible.

Of course, one can return in thought. The interesting thing is that one counldn't even want to return "in fact" if one could not return "in thought" (you have to think it to want it in this sense). And the queer thing about human psychology is that if we can think it, we can want it. Even if it is impossible.