The speaker compares the 24 hours since he has seen his lover to 24 centuries. First he couldn't believe that she was gone. Then he tried to retreat into his memory of her, and then hopes that she will still be into him. He cried a little, sighed a little, then for a thousand years, he couldn't think of anything but her. The next thousand years were just a big vacuum of forgetfulness. But he doesn't want us to think that living 2,400 years in a day has given him "long life." Ever since she left him, he has been a ghost.