The period just after finishing a book is an odd one. You want the just-published book to sell, to break records, to find a wide audience. At the same time, you begin to see ways you might have done better and will do better the next time. You're already planning out the next one.
Writers live this way, poised between just-done and to-do. Only when they get stuck do they teeter for a moment in the immediate present, imaginatively speaking.
A sad fact is that, no matter how close writers come to realizing their vision, most of us are our own sternest critics. It hurts to re-read a book and see only the flaws, the little cheats, the weak spots. We know we should have done better, that we could have done better, if only . . . .
But we can't get anchored to the past, so we go on to the next book (though we do clean up obvious mistakes and typos in existing ones - that's easy with ebooks), vowing to improve. One of these days we'll write the one we want to write, a book in clean shining honest prose, one that will movie and impress readers and that will speak to them directly and powerfully.
One of these days.
Like Fitzgerald's boats bravely beating against the current, we move on to probable disappointment, to likely failing once more to achieve all that we want to achieve. So the spell just between books often finds a writer a little bit depressed.
I think that's a good thing, though. It keeps us going. As Robert Browning said succinctly, "A man's reach should exceed his grasp, / Or what's a heaven for?"