Every time I hand in a piece of copy to one of my clients I think ...
"They are probably going to hate it."
"The last pieces they liked were just luck; this one is crap."
"Oh god, is this really what they wanted? What if I missed something super important?"
Even worse, when I'm working for a new client ...
"They will probably end the contract immediately and write a bad review about me."
That's called the imposter syndrome. It's that feeling that your achievements don't matter and that you somehow tricked other people in liking your stuff, but soon they'll find out.
Scott H. Young published a great article about it.
It's not like I feel like this every minute, but often enough.
I find solace in knowing that I'm not the only person who feels this way. Even the most accomplished people can still feel like failures because the bar they set for themselves is getting higher and higher.
Most of the time, I try to ignore the feeling and focus only on what's right in front of me. I focus on the very next task on my list and on completing it as best as I can.
Because that's really the only thing I can do: Do everything as best as I can. The rest will follow.