According to Robert McKee, the main difference between the professional and the amateur writer is not in their earnings but in their approach to the craft. The professionals know that 90% of the things they write is bad while the amateurs think that everything they produce is brilliant.
Hemingway once wrote in his memoirs that he writes 10 stories and hopes that one of them will be good.
John Green once said that he removes 90% of his first draft and builds from the leftovers.
If an average novel has 60 scenes, it means that you need to write 600 of them and keep the 60 good ones.