Every year, I see the same defeatist attitude creep in during the first half of the month. An author misses a day, realizes that they're now 1667 words behind, and feels like they can never catch up. Many shelve the book after one more day where they don't even break 1000.
If you miss a day, you're not behind. You're just doing what every other author on the planet does: Deal with life.
I talk to authors. Never has one said that they wrote every single day, without fail, wrote the same amount every day, and finished on time. That just doesn't happen. We have good days and we have bad days. We have moments of feverish passion where the words flow like water and when we come out of our fugue, we find that we've added a dozen pages to our manuscript. We have moments where we stare at our word processor and think "I can't do this. I can't look at this book anymore." If we're lucky, we choke out a hundred words before turning on Netflix and breaking out a beer.
In a way, the fact that everyone falls behind during NaNoWriMo is genius. It's an excellent simulation for writing on a longer deadline. Life happens, you miss a day, and then you find the energy to catch up. Maybe you get up early a few days, or resolve to write extra each day, or, like a bolt from the blue, you get inspired and walk away 3000 words richer, and ahead.
So far, I've had two life events hit me: we got a dog, and I hurt my thumb. Dogs are way harder to settle in than cats (especially a dog that doesn't do stairs when you live in a three-level townhouse). You use your thumb way more than you think when you type.
So I'm behind. I'm not freaking out, because I know that I can make that up. I have some ideas coming up that excite me. I commissioned some more blog posts. I have some tried-and-true ways to crank out some words that are cheap and easy. I can get back on track.
And so can you.