It's like the Earth in the picture, just floating through space, la-dee-da, until some huge meteor smacks into it. If the Earth were just a barren rock, then the stakes would be low because the Earth wouldn't have much to lose. But the Earth has spent millions of years growing life from one-celled organisms to sentient beings. It's because of the Earth's success at making living things that a meteor hit would be so unbelievably catastrophic.
It's sort of the same with writing. The more you write, the higher the stakes get because you've invested time and effort into the project. If you don't finish the book you won't get rejected by agents and editors because you'll never get to that point, so it's actually a comfortable place to be. But if you spend all that time writing and editing the book and then you get rejected, it hurts. Big time.
This is where mindful writing comes in. In mindfulness, you need to be aware of the things you can control and the things you cannot. You can't control whether people will love or hate your book, but you can control whether you actually finish writing it. Success and failure are out of your control. What you can control is whether you write the book. After that, all you can do is accept the successes and failures when they come.
What scares you more: success or failure? What can you do today that will take you one step closer to finishing a project?