Just before I had to head out for a family obligation, I was suddenly struck by how amateurish and God-awful the art on the cover of Trang looked, so after obsessing on it all evening, I got back and tweaked it a little and made it smaller. I'm calmer about it now, although I have no illusions about my artistic talents.
It's hard because sometimes those kinds of freak-outs happen for no good reason, and other times they happen for VERY good reason, and good luck telling one from the other. When I applied to graduate school I was convinced that the essay I wrote was the deadest thing in the whole entire world, it just lay limply on the paper like a week-old fish, and it so impressed the person who read it that he immediately picked up the phone and offered me a very generous fellowship. So, you never know--although I write a lot better than I draw, that's for sure.
(And you know how people try to be supportive by saying stuff that's obvious bullshit, like "You'll be an artist someday!" That kind of stuff is really not helpful.)