@enchantedspirit has the same problem despite being an incredibly good writer. When proofreading she, like you, knows what it says and cannot see the error. She sometimes gets me to proofread for her, and I often catch something.
As for your posts, I see your errors but I read through them, that is to say, they don't interfere with my comprehension at all.
I do highly recommend Grammarly as long as you don't take it as gospel. If it flags "sy" in " I say I say I sy" I know I goofed. On the other hand, it insists that center as an html tag should comply with my British English dictionary. Basically, you either recognise the flag as correctly marking an error and change it or you don't and you leave well enough alone. Grammarly does not always know best. It does help me though, to catch most of my mistakes.
Trust me, I'm a doctor.
