The one thing in writing that I wish I had learned even quicker than I did was that, no matter how much you comb through your own writing, it is always invaluable to have a second set of eyes.
Editors are an incredibly important part of several different stages of the creative writing process; they can help you gather your thoughts and ideas into one cohesive story, refine and polish the details of a work, and comb through every line for something as simple as a missing comma. But all of those things contribute to the quality of the work, and no matter who you are, no matter how practiced or professional, that help is needed.
No matter how much an author thinks they can look at their own work objectively, there will always be things that they are going to miss by virtue of it being their own work. As an author you always have context for the things you are writing, but that doesn’t mean that they’ll make sense to an outside observer. Or, you may just miss something completely obvious because you’re specifically looking for it. For instance, there was a story I wrote, and one that my best friend reading it never lets me forget about, where I introduced two characters to one another, and then a few chapters later did the same thing again. In the same story I wrote a scene where a character seemed to materialize into a scene in the middle of the kitchen, where a few paragraphs before I had explicitly put him somewhere else, uninvolved. Even though I probably spend more time editing than I do writing, things in my own work slide past me because I can never completely distance myself from the story in my head.
These were obvious mistakes, and even with as much practice as I have combing through other people’s work, I missed them. Editors are going to catch inconsistencies, not only in plot details but in the details of the settings, the characters, the tone, and the language being used.
Even in grammar and sentence structure there are things that programs are never going to catch: the word that you used three times in two sentences, the awkward way a sentence rolls over the tongue when it’s attempted to read aloud, or an overabundance of adverbs. All of these things may read as technically correct, but any real person reading it is going to notice the strangeness of it (though the person who knows exactly where the issue lies is going to be the one who is trained to do so).
The most seasoned professional authors rely on this just as much as anyone that is just getting started; it is not a matter of practicing until you no longer need this help, but a matter of learning how to work with editors and learn to work their processes in with your own creative ones. Anyone in the business will tell you, it’s not optional. Accept it and utilize it as a strength, something in your corner, because it’s not going away anytime soon.