I'm a professional writer with eleven years experience. I still find errors in my own writing. Sometimes my fingers type too slowly for my brain, and words go missing. Sometimes I copy and paste or revise a sentence and end up with double words. And sometimes I type 'read' when I mean 'reed.' Imagine how much that sort of thing bothers me when I catch it! I
know better, yet somehow the fingers type the wrong thing.
I blame old age.
Although I love revising, I also need a copy editor. I choose beta readers who've got a firm grasp of grammar, punctuation and spelling.