To be nobody but yourself in a world which is doing its best, night and day, to make you everybody else means to fight the hardest battle which any human being can fight; and never stop fighting. e.e. cummings
This is equally true or perhaps even more true for creative people where publising houses, galleries want more of what they have already sold. That makes sense. They are operating a business. The creative person can turn to operating a business also-- duplicate what is already out there. It can be tempting.
As a creative person sees other writers/painters/sculptors/photographers being more successful in sales, or getting nicer reviews-- or any reviews, one of the hardest things is to not want to do what they do. Cummings nailed the problem, but it doesn't make it easier.