Why this is a mistake: Too many beginning writers try to be artists before they master the craft of writing. While in every generation there are some natural-born geniuses who can make that leap, the vast majority of us have to toil away at learning the craft.
There are very few professions in which someone would expect to walk in and be accepted at the highest levels right from the start. Yet, for some reason, people think writing is a profession in which amateurs can enter right at the top levels.
The solution: Learn the craft of writing. First and foremost, learn it by writing a lot. There is no substitute for actually doing something in order to learn it. Then get feedback from those who know more about the craft than you do, either through a network of friends, a writer’s group, a writing retreat, a writing conference, a mentor, whatever you can find. An MFA (master of fine arts) in creative writing is another possibility, although usually the focus in those programs is more on literary writing.