Nope, you're not alone. I sometimes write a lengthy reply, read it, reread it, and before I get sucked into that overthinking loop, press "post" and go do something else.
I then find myself returning to my reply and re-reading it, often catching mistakes in spelling, grammar, or worse, in how I stated or presented my idea. That's why a lot of my replies end up being edited after the fact, with a note saying why I felt I had to edit my response.