Can doubt and boldness coexist?

Whatever you can do, or dream you can, begin it.
Boldness has genius, power and magic in it.
Johann Wolfgang von Goethe
German writer and polymath
How is it that I can start a marathon-like Vineyard Light book project so boldly, then, somewhere around mile 17, get overwhelmed with doubt? Will this project resonate with others? Who cares what a bunch of vineyard workers, winemakers and owners say about life, our planet, our character and spirit?
Forget it. For me, it's so much easier just to start another project.

What suggestions do you have?
How do you keep running with large projects?