Oliver Burkeman has an article on Impostor Syndrome, something that many freelancers, including coaches can feel:
"Impostor syndrome – the feeling that you're a fraud, and any day now you'll be exposed – is presumably even more common than surveys suggest: after all, it's not the kind of thing to which people like to admit."
"The only solution, many experts say, is for higher-ups to talk about their own insecurities much more."
This can be particularly relevant in a world with access to so much information, once a coach gets enough knowledge and experience to understand how complex each individual athlete and situation can be.
"The trouble with the world is that the stupid are cocksure and the intelligent are full of doubt," said Bertrand Russell.
If as a coach you don't occasionally lay awake at night pondering the sessions to be delivered the next day, you be find the quote above applicable to you.
However coach that does feel some doubt, according to this article, it might be a sign the coach is on the right track.