For me it was the sub-district (league) during my senior season of high school XC. The other four guys in our top five ran oustanding races, and there was no reason in the world why the team should not have qualified for districts. No reason, that is, except for the fact that I picked that day to be a colossal f***ing chump. In the two weeks before league I had produced a string of three demoralizingly shitty races and was crippled with anxiety over whether I would manage to qualify for districts as an individual. So when the gun went off that day and things didn't go my way, I punked out like a little bitch and all but dropped out of the race. As soon as I realized that I wasn't going to finish in the top 25 I basically just gave up and let everyone pass me. I finished 55th place in 18+ for 3 miles, having run 16:34 for 5K that year and beaten several of the top 15 finishers.
Anyway, the team scores come in, and our team missed qualifying for districts by 5 points. If I had just had a f***ing nutsack and not allowed six of those slow kids to pass me up, our team would have qualified. Four of the five were seniors that year and I let all of them down. Three of the guys qualified as individuals, but our team captain finished just out of the top 25 and missed his chance at districts and state because of my shit. And I of course missed my chance, having trained all summer with the goal of qualifying for state.
Turned out I had been sick at the time (registered 102 the next day), which explains why I had been struggling. I think my immune system had been weakened due to my constant stressing out about the race.
But there is no excuse for 55th place. That was just f***ing disgraceful. If I had just gutted it out like a man I could have at least finished top 40, and certainly held off six back of the pack JV kids to get us into districts. But nooooo.... I had to punk out like a little bitch with my huge gaping vagina flopped out for everyone to see.
I will never forgive myself for that performance.