The hardest part of writing a novel isn’t the beginning, or the middle, or the end. It’s not getting characters right, world building, keeping your sentences gorgeous, it’s none of those things. The hardest part is having to write when you don’t have the heart for it.
When you’re sad, or distracted, or in a bad mood, or bored. It’s writing when you can’t think straight, when the words are arranging themselves in dreadful “sentences” that hurt your brain. It’s writing when writing is the last thing you want to do, and every word, phrase, sentence is a struggle.
Writing through a crap day is the very hardest part of being a writer. Then getting up the next morning and doing it again. And the next. And repeat until the bloody book is finally finished.
(Blurbs are still harder, but.)