It's dangerous business. I often look back on some blog post written late or in a hurry and find the most ridiculous mistakes. I mutter under by breath, fix what I find, and wonder what I have let slip by in other posts.
In this age of self publishing, which is all the rage, don't self edit. You'll regret it.
You will miss certain misspellings because your mind will rearrange the letter into the proper order, whether they appear that way on the page or not. That's why you so often see "form" where "from" was intended. You may type "our" when you meant "are."
Then there are the mangled sentences. You were in a hurry. One sentence sucks, but the first half says something valuable. You cut away the worst and apply the second half of the next sentence to it. You move on, missing the fact you've left two verbs side by side. That's only one example of a cut and paste sort of error.
Then you write late at night. Some sentence goes by and it makes no sense whatsoever. If you don't have an editor to ask you what the heck you meant by that, it goes out into the world. Suddenly your readership wonders if you aren't off your meds.
Let's face it. We all make decisions about other people by the way the write. Without an editor, readers may unfairly view you in some negative way that would never have happened if you'd had an experienced person lending you a helping hand.
Additionally, there are a whole lot of rules you know nothing about. Rules of punctuation and sentence construction that change with remarkable and irritating frequency. Worse, there are a great many rules concerning copyright law that an experienced editor will know (not to mention libel law ... brrr) that you simply don't. Not knowing that sort of thing gets expensive.
Final word: if you are getting professionally published and you have an editor from the publishing house DO NOT turn the edited manuscript from that professional over to some editor on the side you used before you turned in the manuscript to the publisher. Most likely, that side editor will be using a different style manual from that of the publisher, AND will get defensive about the number of changes the pro made after the side editor finished ... and that always ends in bad language and tears. Don't do it.
Best of luck with your writing.