Beginnings are hard to write.
Endings are even harder to write.
But the hardest part of a book to write, hands down, is the middle.
The middle of the book is that part where ennui sets in, the part where you start to heartily dislike most of not all of your characters, or — if you still like them in spite of everything — the part where you become so tired of the fictional milieu you’ve embedded them in that you start to fantasize about lifting them out of it wholesale and giving them all jobs in a coffee shop instead. The middle is where plots break down, where minor characters show up out of nowhere and attempt to hijack the narrative, where major characters suddenly take left turns into unmapped territory.
Sometimes the plot breakdown is obvious when you hit it, and you end up stalled for days or weeks or sometimes, heaven help you, years, until you work out what’s holding things up. Other times, you don’t notice it until it’s time to do the revisions, and then you’re stuck doing a massive structural rewrite on a short deadline.
One way or another, with novels it’s the midgame that makes or breaks people.