I met Vern at a writer's conference and he attended every year. He had an excellent nonfiction book, but he had never written the final chapter. Editors had read portions and wanted the book, and so did several agents. But Vern never finished.
He knew what he wanted in the final chapter and he told the editors and agents, but he couldn't write that final chapter. At the time, I thought Vern was unique; he wasn't. I hear this regularly from people who get thumbs up from editors or agents and never finish.
Vern finally told me why he couldn't finish—why he resisted. "I was depressed before I began to write. That depression went away while I worked on the book. I'm afraid that if I finish the book the depression will return."
I started to reason with him and stopped, because I sensed that anything I said wouldn't help. He'd probably heard all the arguments and suggestions before. I reminded myself that Vern had to make up his mind if he would allow his fear of returning depression to stop his finishing the book.
I have another talented friend who has started 15 books (and possibly more). He has never finished one. His reason isn't the same as Vern's but the result is.
Some writers are afraid to finish a writing project.
They allow their fear to block their creativity.