Some writers get the jitters after they've finished a book. Galleys are done, review copies out. No going back. Too late to fix the horrible errors that are certainly there. Readers will hate it. Reviewers will shred the book and crucify the author.
My jitters come earlier in the process. As a writer incapable of using an outline for fiction, I begin my stories with a barely-defined character in a murky situation, turn the character loose, and start banging the keyboard. My first drafts are masterpieces of literary malfeasance, the poor characters stumbling this way and that, trying to find their way through the novelistic equivalent of a classic London pea-souper.
As I wrote my first few books, the end of a writing session found my characters and me equally exhausted. But I learned that if I'd just...keep...writing, the characters would gradually reveal and define themselves. Words I couldn't have imagined would be spoken; deeds I couldn't have envisioned would be done, and the book would spring to life. Like a biologist bringing a slide under a microscope into focus. By the time I write "The End" - well, I never actually do write "The End," but to make a point - I feel as though I've told the story I wanted to tell (which is always different from the story I had thought I'd wanted to tell), and that I've done it as well as I could. By the time the book comes out, I'm into the next story*, and any jitters I have are related to the new project.
Sometimes I think about all the other roads my characters might've taken, but didn't. Just like real life. Alternative histories haunt us as we constantly make choices that send us off in particular directions, too bad about the myriad other possibilities that never will be.
To take it a step further, how many other possibilities were there at the moment my parents did what it took to put me into the world? I'm sure there could have been better Larry Karps, and I guess there could have been worse, but for better or worse, here I am. All I can do is keep stumbling forward, and hope that when I've finished the final draft, it will be the best story I could have told.
* There are exceptions to every rule. With A Perilous Conception out for two months now, I'm not yet into my next story. But that's a whole nother story. I hope to be generating my next set of jitters soon.