Quick note, mostly to future self: the middle of anything long always feels like it’s failing, and this feeling is not evidence. You can see every flaw at that point because the flaws are the only finished part — the fixes, the parts that will retroactively make sense of the flaw, don’t exist yet. Of course it looks worse than the finished thing. It’s not the finished thing.
I keep forgetting this at almost exactly the two-thirds mark of everything I write, on a schedule regular enough that I should trust the pattern more than I trust the feeling. See this note for the borrowed version of this same thought, which says it better than I just did, from someone who apparently had the same problem in letters a century earlier.