But very few bits of writing advice tell you anything at all about how to write last lines. I think they think that by the time you reach the end of your story, you should know how to make the last stroke reverberate. Or else it's that grabbing someone is easy, while satisfying the weight of an entire novel is hard.
Anyway, every novel I've ever written has held me in minor terror that I will not be able to write the last line. I have always been afraid that all of my out-of-sequence writing will come down to the last line of the last paragraph of the last chapter, and I still won't know what to say there.
This has never once happened, and today it didn't happen again: the last line of What We Did to Save the Kingdom is, "There was much to do." It may not stay the last line through revisions, but it's a last line, at least, and it will serve if nothing else does, and I keep writing it when I'm writing notes about this book, so I started to think maybe there was a reason, and maybe I should go with it.
There's still a bunch more novel to write here -- there is still, in fact, much to do -- but the last line isn't hanging over my head any more, and that's of no small comfort to my brain.