There's a bit of software, in your head and mine, that boots itself up
JUST AS YOU ARE ABOUT TO FINISH.
I am finishing two book manuscripts and the software is working right now. I sure don't want to arrange the book (chronological order? thematic? other?) or write up that table of contents and acknowledgements page. I think it'll take up too much paper to print out the manuscript for proofreading. I'm not thinking, "How great it is I've finished a book, or nearly." I'm thinking, "What awful things will people say about me when I publish this book?"
The software kicks in just as you are about to succeed. In fact it signals that success is near. But suddenly you're exhausted or depressed. The world doesn't need another writer. In fact, you need to train for a triathlon. (Note: The world doesn't need another triathlete, either.) You quit your writing group, thinking they don't respect you, they never did. . . It's not writer's block. You can write just fine; you just can't finish.
Don't try to pep-talk or bribe yourself. It won't work. Get help. Ask a friend to help you scout a bookstore for names of likely publishers. Ask a fellow writer for encouragement or to set a deadline. Pay someone to write three query letters for you. Talk to a businessperson about business; this can help you lose your fear of it. Tell a therapist, if you can afford one, that you have written a book you just can't bring yourself to finish.