Doubt is one of the biggest challenges you'll face as a writer. Doubt that your writing is any good. Doubt that your work is ever going to get published. Doubt that, even if your work were incredible and published, there would be anybody in the world who'd want to read it. You're not alone, and though I can't definitively say that every successful person has faced these doubts, I believe most sane, rational people have, even when they are on the right path, making all the right choices. The trick is not letting them overwhelm you.
It's human to have doubts. It's rational. And when it comes to tackling a project that will take months or years before it's ready to share with another human being, especially if it's an ambitious project, doubt should be expected, even accepted. Then, just as certainly, it needs to be cast aside. Tossed.
Doubt is just another word for fear. It's the fear that what you're doing is a waste of time. But writing is never a waste of time, even if it ends in failure. Cormac McCarthy famously said, "Even if what you’re working on doesn’t go anywhere, it will help you with the next thing you’re doing. Make yourself available for something to happen." I have this quote on my wall, next to my writing desk. When I have doubts about what I'm working on, I go back to it. I think, So what if this project collapses before it gets off the ground? I'm writing.
Writing is not always easy. You'll hit roadblocks that will make you question your project. A work-in-progress can twist and turn and lie down limp and jump up and smack you in the face. All you can do is keep writing, plotting, working around the challenges and working around the doubt until it's no longer valid, because suddenly, the project that you doubted would ever come to completion is just a few thousand words shy of the climax, and then you're writing the last line, and when you read over what you have and see how clunky and unpolished the writing is, you're smacked again with doubt that this was worth your while, but you push the doubts aside again and you rewrite the first few chapters and the language is suddenly alive, and the characters feel like old friends, and you can't believe you ever doubted yourself.
I should also say that some doubt can be a result of putting too much pressure on yourself. Maybe the pressure comes in the form of a self-imposed deadline, or from comparing the writing in your rough draft to the writing in your favorite author's best book. Finding a way to ease the pressure can help alleviate the stress that comes with it, and that can allow you to have more fun with the story and give yourself over to the writing. Try to forget the world around you when you sit down to write. The act of writing can and should, at least at times, be a reprieve from the stresses surrounding it. It should be fun. It should be freeing. Sometimes.
So, when you're in the midst of those common doubts, remember that writing is all about process. Doubt might be part of that process, but if you play it loose and allow yourself to forget your career aspirations or your deadline or whatever, you'll likely have an easier time letting go of doubt and finishing the project at hand.