Write a “Lite” Version: Have Fun and Be Playful.
You procrastinate because you want your book to be successful. You fear failure.
Amazon lets you publish your book fast. So, why not create a “lite” version of your book? This won’t be THE BOOK, the one you’re procrastinating on. It will be a version of your book. An experiment. It can also be your own private focus group.
For years, I’d advise authors who were unsure about a book to create a blog on their book’s topic. If their blog got attention, their book would too. On the other hand, if their blog was completely ignored, they needed to rethink their book. And yes, this worked for fiction too.