In 1973 I began work on a story that for years was a work in progress. The original was written long-hand and was rewritten many dozens of times over many years. By the 1980s it was close to 4000 pages long- a massive tome. Thinking I'd written a modern masterpiece, I handed it over to a publisher for consideration, certain of success.
The publisher in question was kind enough to read the manuscript, and got back to me in due time in order to inform me that the work was a complete mess, the greatest problem being that the work actually consisted of several shorter works. I spent years prying my tome apart into its smaller constituents, with varying degrees of success. These became Journeys (unfinished), Ralph and the Pixie, The Dark Magi (unfinished), The Wizard and the Sylph (unfinished), The Travellers (unfinished), and enough leftover material for several other projects.
Some of these stories remain unfinished because pulling apart the original meant unravelling the overall storyline, leaving a lot of work to be done in order to make each a stand-alone project.