A work of art leaves the nest / studio / computer when it is fledged, and it never returns. It becomes the past, something one worked on, but the connection is snapped. The hot sensation of being in the middle of a project is gone.
The only cure for the sense of loss is a new project. As Sam Sloan says in "The Malted Bunneh", "I have some ideas ..." The next painting / movie / whatever is always the one that captures the imagination.