From On Writing, a book I would recommend to all prospective writers.
I often need to remind myself of this. It’s easy to be confident in your abilities when you’re a teenager and pretty much the only serious writer in your high school. Then you grow up, and you’ve been slogging away at this craft for more than twenty years, and you’re struggling to just get noticed (you and every one of the other 3 billion indie writers on the Internet), and you have to keep telling yourself that giving up is not the answer.
And also that “I’m just fooling myself” goes both ways. Sometimes – often, most likely – you’re doing better than you think you are.