I took time off from the book for a couple of days to work on a side project. I also needed a couple of days to think over part of the storyline for the book: this kind of midway reassessment used to send me into a panic that on some fundamental level a story wasn't working, but now I recognise it (having the experience of three previously completed books, two published) as pretty much par for the course. I get to fifty thousand words or so, and I know which parts of the story work, and which parts might look good in synopsis form, but don't work nearly as well when it comes to incorporating them into a fleshed-out narrative.
Basically I'm thinking of chucking out the entire backstory for the main character: how they got to be where they are, told through a series of flashbacks that details the how and why of who they are (if you've read Against Gravity, I did the same kind of thing there with the sections showing Kendrick in the Maze). This won't affect the rest of the story, since my protagonist will still end up in the same place regardless: it's just how they get there I'm thinking about.
So I spent part of this morning writing out and revising a new back story for my protagonist, based on some ideas that have been swilling around in my head for the past week or two. What's coming out feels like it fits a lot better than what I previously had, and again it just feels par for the course: I usually find it's not worth me working out too heavily detailed an outline prior to starting a book as I always find a lot of really good ideas come to me while I'm actually typing prose to screen. These always end up spinning the story around in some new direction, usually a good one.
It's worth mentioning all this because I'm entirely prepared to believe I'm far from alone in that kind of midway-through-the-story panic, when you can start to feel lost in the details. I suspect it's the thing that stops a lot of people from finishing those unfinished novels: what feels like a good idea at the start becomes mundane from overfamiliarity, and you start to lose confidence. The trick is to not forget what a good idea it was at the start, and to be prepared to reevaluate what you have at key points through the process of writing - meaning, usually, roughly the midway point. The nice thing about this is, unlike with Angel Stations and Against Gravity, I'm actually enjoying making the adjustments rather than tearing my hair out.