It seems what you are really asking is how self-pubbed writers get the errors out of their manuscripts.
I can't speak for everyone, but I can discuss my process.
First of all, it's important to have an excellent command of the English language. I understand things like what participial phrases are and how to use commas correctly with introductory phrases. Initially, my knowledge was a bit fuzzy around the edges in places, so I actually studied things like commas and apostrophes until I could write intelligent blog posts about them. I'd recommend polishing your skills in this area. Even though I felt I was quite competent going into this process, I learned a lot doing it. If you can't clearly articulate why you do something, grammatically speaking, then it's possible to benefit from study in this area.
Secondly, it doesn't sound as though you're reading your words aloud. I do mine at a whisper to save my voice, and then I have Word read back the entire manuscript as well. That covers a multitude of errors. You'll also catch things like unintended rhymes this way.
Then I do beta swaps with extremely talented people, most of whom I met on AW in Share Your Work. They usually spend their time on line edits and cutting things like excess world building, but an occasional proofreading blart gets fixed this way as well. I do this multiple times for every book.
Then I always buy a kindle copy as soon as I publish and read it. It's amazing how a change of format can help you catch things.
And then I get my initial book reviews on goodreads, swapping in the moderated author review forum, and if any errors have made it through to there, usually a reviewer is kind enough to point them out privately.
Now and again I'll see someone on AW advocate paying for a full-on trade process, and I think that's a way to go deep in the red. My process works well for me, and many SP authors do something similar.
Do what works for you, but in general, I'd say it's better to build skills than to pay money. Beta swaps are an intense skill-building endeavor. If it weren't for my beta friends, there's no way I'd have ever had the courage to self-publish. Thank you again to everyone who's helped with this. : )