Every session. To me, it's an opportunity for improvement; I think about it right afterwards (couldn't stop myself if I tried). Write down notes for how to fix it next time (new thing on screen, or better process, whatever.) I find that flushes it out of my system afterwards.
I do find that teaching college (my money job) is a lot like DM'ing in some respects; part performance, part technical, prep time, etc. I've been doing this almost daily for about 5 years now, and do the same thing -- screw up, make notes, do it better next time.
Oddly enough I have
just gotten to the point (like within the month) where once in a while I have a day where I
can't think of anything I screwed up. In some ways that actually worse! I head home my brain gets caught in a loop: What'd I screw up? First class, second class, third class? Nope, nope, nope. So what'd I screw up? And I wind up not getting to anything else and sleeping weel that night. Who knows, maybe I'll start getting used to it.
