I belong to several writing groups. In one there recently was a woman who asked for help during the editing process. She wasn’t able to see her own errors.
This is a problem that many writers have to overcome. It has to do with sinking into the world you’ve created too far to see the actual words you’ve used to get there.
I believe the trick to editing your own work is 3-threefold.
First time is for continuity. Has your hero mysteriously changed eye color? Did a location suddenly grow a room? Have you forgotten a crucial plot point?
Second is paragraph by paragraph. Does each paragraph enhance the story? Does it convey the feeling, thought, idea you intended it too?
Third is line-by-line and it’s as tedious as the name implies. Every single sentence is examined for grammatical errors, punctuation, and the ‘dreaded’ adverb.
Does this mean you edit the complete work 3 times? Yes, at the very least 3 times. Some authors do many more than that. David Farland has said he generally does 8 edits before he’s happy.
Not that your manuscript will be done at 3 or even 8 edits. It’ll never be done to your satisfaction. But 3 or more edits will provide you with a document you won’t be ashamed to send out for outside opinions.