Get through the confidence gap, but have the courage to quit
I like this a lot. However, I do believe that it is okay to quit sometimes. Maybe you don't have the energy or talent to become the tennis player, writer or musician you want to be. I hesitate to say this because, inevitably, some people will give up too easily.
As IG says, you have to practice. If you get no joy out of practicing, you'll just be frustrated. This can take courage to recognize. Be as brave about quitting as you are about not quitting.