I'm sure it's happened to all of us: you're well ahead, confident of victory, and dividing the remaining few points in yose. Suddenly your opponent makes a move that targets a hitherto-unseen weakness and you realise one of your big groups must die, and your chances with it.
This happened to me today, and to make it even worse I made the wrong reply. If I played correctly, my opponent would have captured eight stones, but the remainder of the group would have lived, and I would still have won comfortably. In trying to save the eight stones, I lost the whole 25-stone group and with it the game.
It's hard to convey the sick feeling of disappointment and self-criticism you feel in this situation. Being beaten when you played well is manageable, but giving away a won game with a single silly mistake is crushing. It's at these moments that we feel we never want to play again.
They say 'Experience is what you get when you don't get what you want'. In other words, winning a game teaches you nothing. If you lose and you can work out why you lost (in my case, it's embarrassingly easy) then you have just become a stronger player.
So why do I feel so bad? My opinion of my own strength has collided abruptly with reality and I'm forced to recognise that I'm not so good as I thought. This, too, is helpful. I like that Go teaches you humility — if you weren't humble to start with, Go will show you the folly of your own pretensions. If you were fairly humble already, Go will keep you that way.
It's tempting to just curse yourself over and over for such a gross mistake. But that achieves nothing. In a way, you're trying to shift the blame to some other part of yourself, so that it doesn't reflect on the 'real' you. "That stupid Go-playing part of my brain. The rest of my brain would never make a mistake like that! We want a divorce!"
Life is full of mistakes and disappointments — not quite full, of course, as there is still plenty of room for love, joy, and success — and how someone reacts to them is an excellent guide to their character. The Japanese say "Fall down seven times, get up eight". Someone who can learn to do this will, in the long run, be a winner — and not just at Go.
"He who gains a victory over other men is strong, but he who gains a victory over himself is all-powerful."
— Lao Tzu