You're not afraid you won't/can't finish writing the book, you're not even afraid it'll suck, you're afraid no one will care. You're afraid it doesn't/won't/can't ever matter. You're afraid you've arrived so late the party is over and no one told you where the after-party is cuz you're not cool enough. But here's the thing: Yours matters as much as any book ever written if it's genuine, if you believe in what you're doing. There is no "good," there's only true. Write true and it'll be worth it.