First, writing is not an exact science unless your subject is very exact and like a science. Because of this there’s no exact way to know you’re doing it right and certainly no way to know you’re doing it perfectly. This makes it easy to imagine all the things you could do differently and never think of “right enough” – or developing your own standards.
Secondly, writers are imaginative. We can come up with all sorts of ways to decide how bad we are. We turn imagination on ourselves.
I was talking before about how we need to think of creative work in terms of palettes and not hierarchies, and Steve's notes feed back into that line of thinking.
When we create something, anything, it's hard not to think of it in terms of how it competes with what other people are doing. In a sense this is not wrong, because at the end of the day it is competing with other things — it's competing for a slice of someone's time. There's more to watch, read, play, and experience now than ever before, and so it's inevitable that the overall audience for any one thing is going to be smaller than it used to be. Nobody would blame you for wanting to grab as large a slice of that pie as you could make off with.
Thing is, seeing competition for an audience's time as the ultimate goal only makes sense if you think of creative work as a zero-sum game. It's not. It can sure feel like that, when you feel like you have to compete against Marvel blockbusters and AAA-level video games. It's easy to feel like an impostor when you try to produce something on that level — something that can only realistically be achieved with tons of money and thousands of people.
But people can make as much space in their lives for the little things as they do the big ones. Find the audience that cares about your work because it's your work, not because it reminds them of other, better things. That's one way not to feel like an impostor.