I like to let my work sit. Not all writers do. Since I have multiple projects (including novels, short stories, non-fiction, and poetry) on the go at any one time, and I'm always ahead of schedule, I can afford to do this. I typically let my work sit for six months. This gives me time to rethink what I've written, but having a time limit on how long a book or poem is allowed to sit for keeps me from toying with it needlessly (and forever).
When I do revisit my older works, I inevitably find things to change. This is great. It means that the book, poem, or article will go off to the editor/publisher as polished as possible. I'm finding things that weren't perfect the first time around and making the entire work better. There can't be anything wrong with this. Can there?
Well, let's talk about the torment of the entire situation. If I read a sentence and can't believe I wrote it, it's a little bit of a blow. Not a big one. I'm a writer, I can cope with changes (whether those are made by me or someone else). Still ... the idea that I could write a sentence that isn't perfect is annoying. Yes, I know perfection is unattainable, but that's not the point. Revising is still not a lot of fun.
I typically revise for three weeks. That's it. I don't make any changes after that unless asked to by the publisher/editor. I should be happy, right? I am. Right up until I read my own book a year later. I always find something I wish I could change. A piece of plot I didn't include. A minor character I didn't flesh out. A word I wouldn't have used. Dialogue that could have been wittier.
In most cases, the work has already been published and it relatively unchangeable. Even if I could change it, it's not usually a good idea to make significant changes. So I have to live with an imperfect work. And that can make me just a little sad.
But there's a bright spot in all of this. When I look back on books written three or more years ago and I note all the things I'd change, all the things I'd do better, I'm heartened. I've progressed as a writer. I'm better at my job now than I was three years ago. And three years from now I'll be better at my job than I am now.
I'm growing as a writer. The torment of knowing that there are things I'd change in past books is lessened by the knowledge that each subsequent book is better than the last. And this is a wonderful thing.