It’s a common feeling and it’s called “buyer’s remorse”. Many times it happens well after the actual purchase as the thrill and pleasure of possessing the item fades and the price looms larger. Quite a lot of advertising is directed at buyer’s remorse. It’s awfully hard to make you buy a car, eat a burger or drink a beer if you’re not in the mood, but the ads showing happy, content people, can make you feel better about the purchases you have already made.
I experience plenty of “writer’s remorse”. When I look back at my works, I am often plagued by thoughts that I could have explained this better or developed that character more or edited that scene further. It’s a cross between perfectionism and insecurity (those two tend to feed into each other). I can’t speak for other authors, but for me, it’s much more annoying than buyer’s remorse, since the object in question is my own creativity.
Fortunately, writers have something similar to advertising to help assuage our fears: reviews. Whatever misgivings I may have about the published work, I can rely on the opinion of the people who actually paid to read the book. The ones that post a review or email me directly are few in the overall pool of readers, but their feedback is invaluable. I am fortunate that (to date) the reviews and feedback I have received are positive. That’s the only way I know to get over my writer’s remorse.