Some people never get around to writing because they don't know whether their idea is good enough to succeed in the marketplace.
Welcome to the club!
Every book, poem, short story, and screenplay is a gamble. Even the top publishers are not very good at predicting which books will take off and which will have disappointing sales. The same goes for movie studios, hence the famous William Goldman quote, "Nobody knows anything."
Sometimes writers have a huge success with one project and a huge flop with the next one, or vice-versa.
All you can do is your best. Write the book or script, do as much as you can to help make people aware that it exists, and move on to the next one while still devoting some time to marketing your previous projects.
