I can read a sentence someone else has written and see every glaring error. When I read my sentences, I know what I meant, so typos, word choices, grammar, even incorrect dialog attribution goes right over my head. It is nearly impossible to do the final editing on your own story. So what if I miss a few little things among 100,000 words? Some readers will be driven crazy by it and never read another sentence, let alone buy another book from me. No big deal? They will write a bad review. Big deal.
I have several strategies for self-editing. First one is to marry someone that will do it for free. Not an option? Software. Grammarly is free, but it is tedious. The Editor in MS Word is “free” if you wrote your story on the expensive subscription Office 365. There are other free or inexpensive online editors, all with good and bad aspects.
The problem with these tools is they will tell you things are incorrect when they probably aren’t. If you trust them too much, the story will lose some of what makes it yours. The best way to do things is to develop a system. I will detail my current system of proofreading, but it changes after almost every story I edit. This is because new tools become available and old tools improve.