I know that probably makes you just as cross.
Not really. You're a good writer without a beta, you at least acknowledge that you could be better-don't get me started on the egomaniacs who think they're Tolkien when really they make Stephenie Meyer look good-and, while I've never actually seen your reaction to this sort of situation, if someone pointed out a mistake you'd made, I'd bet money that you wouldn't throw a hissy fit. That already puts you heads and shoulders above pretty much every single person my rant is directed at.