At a book signing, someone asked me "what was the hardest part of writing this book?" I didn't have a great answer at the time, but I've been thinking about it ever since.
The hardest thing might simply be having faith. Faith that, if you devote a good chunk of time, energy, and thought, every day, for many, many days, you will eventually end up with something.
It's relatively easy to keep the faith when things are going well, when you're in the flow of your work, when you're feeling confident. It's harder when you hit a rough spot. In that regard, writing is no different than anything else.
It's hard to believe in yourself. And it's hard to be selfish enough to keep at it. I always have doubts - especially when I'm sacrificing financially and postponing things that need doing around the house to spend my time writing. That's why I'm so grateful to have family who have been unfailingly supportive, and friends who say, without a hint of skepticism, "wow, you're writing a novel? That's great!"