Change the question from, "Is this book any good?" to "Did I hit a reasonable word count today?". Writer's block is grandiosity and self-importance run amuck. If you're suddenly convinced your book is terrible and it's going to destroy your career as a writer, those beliefs presume a large number of people are actually going to read it once it's finished. Some of the most exciting and rewarding projects I've done seemed terrifying at the outset so I just convinced myself no one was going to pay attention to them once they we're done, and that's how I found the courage to take the leap.