I just started reading Hardwiring Happiness, by Rick Hanson and got to a section on the negativity bias and how it shapes conscious experience based on evolutionary past. It's an idea I've seen before in looking at psychological biases and elsewhere in printed material from Daniel Kahneman. Essentially, that from an evolutionary standpoint, it was much more important to avoid negative experiences (ie. death by predator, dangerous areas, poisonous food, conflicts, and the like) than to find positive experiences. While both were important, the avoidance and vigilance of negative experience shaped the brain in such a way that we are wired now to consistently value negative experiences stronger in memory.
As Hanson writes, if your boss gives you 20 compliments and one criticism, it's likely the criticism to stick with you. Similar examples abound in life; we are stuck, evolutionarily speaking, valuing negative more than positive in shaping experience. I remember one time I had immense and painful stomach pains immediately after having a protein shake made from pea protein. For a year or so I had been fine drinking the same protein shake, but that one bad experience made me throw away the rest of the powder and never have pea protein again. I'm sure you've had some experience like that with another food. I'm apt to weigh negative more than the hundreds of protein shakes I'd already had that had no ill effect on me.
I think some of this carries over into every domain of life, and I wonder exactly how it plays out for the athletic self. Controlled pain is almost a part of growth, but we don't really view that as a negative experience. If I hurt myself squatting, I may come to the conclusion that the whole way I was squatting is wrong and I need to change/fix everything. It might just have been the case that I was a little loose in the movement, or some muscle was a little tighter than usual.
A single bad rep or attempt in competition may weigh more heavily on us than the hundreds of reps that preceded it, causing an almost myopic focus on the wrongs we did, instead of celebrating all of the successes. It's hard to say "hey that's just your evolutionary biology at work" but I really think a large piece of it is.
Hanson posits some ways to change this, and you're welcome to read his book here.