Sometimes you don't...
Sometimes you don't know if you're doing the right thing until you've done it. I've never refused an offer to publish a book until I turned down an offer from the publisher of my last novel to publish The Winged Child, the book I've been working on for the past year.
I didn't know why, but I just couldn't feel right about it, so I let the days slip away until the deadline passed and the publisher voided the contract. While fretting over it, quizzing myself as to whether I'd done right by my book, I pulled out the manuscript and began to go over it one more time.
I hadn't made many changes before I realized that the book really needed some rethinking and reworking. If it had been published as it was, I would have been dissatisfied with it no end. It was a good book when I submitted it, I think. But it wouldn't have been the right book.
Maybe the winged child will get another chance to fly. Maybe not. But eat an apple before it's ripe and it might leave you with a bellyache.