It's part of my nature to dwell on any mistake i've made in a competitive field.
If I didn't play to my full potential then I feel i've let myself down. If it's a team game, I dwell even further on the little things that may have made a difference on the remainder of the match.
These can be little things sometimes but they can really depress me for the rest of the day and possibly as long as a week if it was significant. I suppose it's a good thing in the sense that I aim to perform to a higher standard in the following game (whether it's football, rugby, poker) but i'm not a guy to be around when i'm in one of those dark moods.
Worst situation for me was in my final game of senior school rugby. I'd had a glorious season and generally carried the team through many a game. The last game of my schoolboy career saw us get absolutely hammered. I was completely over-run at full-back including one horror moment where I was set to chase a kick over the top only to slip and let their fly-half in for another try. That mistake earnt me "dick of the day" and capped off a shocking climax.
That day saw me sitting alone, crying, at the front of the bus.
Emotional day indeed.