There is no writer on earth who can effectively edit his or her own material.
I could have stopped this post right there, but the self-evident truth of that statement is probably quite a bit more self-evident to someone who has spent nearly all of his or her adult life writing and editing things for publication, as I have. So for the benefit of those without professional publishing experience, we will dive into why this is so.
Writing is in many ways a conversation with oneself. The words form in your head, and must pass out of your brain, down to your fingers, past the keys, and thence onto the screen. That’s quite a few intersections where you can unintentionally t-bone an impulse going a different direction. The problem is that the piece being written on the screen is being simultaneously etched into your memory, and in the process of re-reading your work, very often that internal text overlays the external one enough to conceal errors. You think you’re reading what your eyes see, but in fact you’re listening to your brain read that text in your head. This is why you can read over a text and not see a typo until you’ve read it 10 times or more.
Someone else reading that piece has no internal text to trip them up, so they are more likely to catch these errors you miss. This is why you should never, ever publish something that has seen only one pair of eyes.
Consider for a moment what happens with a book that sees print publication.
Most experienced authors of fiction have developed a habit of getting someone—very often their spouse, but sometimes a friend or two—to read their work before anyone else sees it. Not all authors do this, but the successful ones almost always do. When the book is submitted to an agent, it is often read first by the agent’s manuscript readers. Most agents get far too many submissions to read everything that comes across their desks, and so employ readers to sift out the gems. If the reader recommends the book, the agent will then read it. Should the agent decide to represent the book, it will then undergo a comprehensive proofread and copyedit by the agent, and—depending on the size of the agency—one or more copyeditors before it gets submitted to a publisher. What happens upon submission depends on the size of the publisher. There are small indie publishers in which the owner handles everything, but with larger publishers, and especially the big ones, it will be read by at least two or three acquisition editors before a decision to publish is made.
In the event the publisher decides to pick up the book, it will then be assigned to a managing editor (who may or may not be the same person as the acquisition editor), whose job it is to shepherd the book from manuscript to bookstore. At this point, it will undergo another comprehensive proofread and copyedit by the managing editor and possibly several copyeditors. This process will be repeated at the manuscript stage, the galley stage, and the final proof stage before anyone has a chance to actually buy the book.
What this means is that the average book on the shelves at your local bookstore, as well as the ebooks from the major publishers, may have been read by upwards of 20 people—all of whom make their living preparing things for publication—before you get your eyes on it. That, in a nutshell, is why it looks so polished and professional.
Compare this to the process that most self-published erotica goes through. The author writes it, slaps together a quickie stock image cover, converts it to ebub or mobi format (or maybe just uploads it to Smashwords as a Word doc), and that’s that. This, in a nutshell, is why stuff like this looks so amateurish.
Self-publishing has its virtues, to be sure, but cutting the professional editors out of the process is not one of them.
What can you do to avoid this problem? If you can’t recruit a spouse/significant other to read your stuff beforehand, you can at least recruit advance readers to help out. When I was writing for the Usenet years ago, I accumulated a stable of seriously nitpicky proofreaders who were happy to proof my stuff in exchange for getting to read it before anyone else did. Making this happen takes time, but the effort is worth it.