As a former English teacher and writing/grammar tutor, I consider my grasp of grammar fairly good. Probably even a little above average (at least better than most of my former students when I got them).
Still, I have always struggled with homophones. It’s not that I don’t know the difference between their/there/and they’re, because I do. Just as I know the difference between affect/effect and all of those others.
Usually I catch these mistakes in editing, but it is a consistent problem that I have. I’m not sure where the disconnect lies since I know the difference. Still, it’s very frustrating to make the same mistakes even though you know how to fix them.