This happens every time I write a novel. I get halfway to two-thirds through it, and I realize I’ve gone so far off my original outline I have to sit down and rewrite the outline again from that point on. Sometimes I have to go through it twice in a single book.
This isn’t a bad thing. As I write the book, I come to know the characters better, and the situations they’re in. Things happen I couldn’t have predicted when I wrote my first outline. It’s far better to flow with the new notions than to try to force myself along a path that’s not faithful to the story as it’s developing.
Still, when you’re writing under a deadline, the idea of having to stop so you can reassess where you are and where you want to be heading is frustrating. However, I’m confident that it always ends up being a better book than I could have hoped for at the start.