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'The second you think you are doing well is the second you are finished.'
This is so relevant I shouldn't be giving an explanation. I will give one, because I want to, but I shouldn't have to.
Hands up if you have ever lost a game you (thought) you had won. If your hand isn't up right now, I think you might not be human.
We've all had that game where everything just goes right. You're in a good mood, your opponent is challenging, but not better than you, and you are winning. Nothing could possibly go wrong. Right?
Usually this is where I lose focus on the game and lose. Either I was really winning and made a (stupid) mistake, or I wasn't winning at all, I was just being delusional. One way or the other, I get overconfident and lose.
It's not a bad thing to know if you are ahead or not in the game. You should count during the game! But when you then find out you are winning, getting overconfident is pretty much the worst that could happen. It makes you greedy, unfocused and generally not a nice person.
Overconfidence = bad. Stop it.
Which is a lot easier said than done.