Harry Potter becomes quite upset at the concept of being possessed by Voldemort during his teenage hormonal phase in book 5, at least until Ginny reminds him quite sharply that she's been there and done that, and it's not at all like what's happening to him.
In the last book, Remus Lupin offered his service toward Harry Potter while at the same time, as noticed by Harry, leaving his wife and unborn son, believing that they will be better off without him. In response, Harry (who is an orphan, and had gone through miserable treatment from Dursleys), calmly and viciouslypoints out his cowardice and preference to "die in glory" than become a good father. Of course, he's snapped, but at least it had the desirable effect: he returned to his wife afterwards.
Happens at least once in Dragonquest, when F'lar thinks he's about to die.
God often sends his angels to give words of encouragement in The Bible, but on the top of the list of epic "Get A Hold Of Yourself"s from God Himself is when Elijah has a Heroic BSOD. Full text here, but here's a sample:
"Then a great and powerful wind tore the mountains apart and shattered the rocks before the LORD, but the LORD was not in the wind. After the wind there was an earthquake, but the LORD was not in the earthquake. After the earthquake came a fire, but the LORD was not in the fire. And after the fire came a gentle whisper."
In Dave Barry Does Japan, the author receives a big SLAP (possibly from himself) when he goes off on a long rant about American bureaucracy.
In The Abhorsen Trilogy, Lirael has a dogbite-shaped scar on her leg from when her wangst was getting way too out-of-control and the Disreputable Dog decided to intervene. After that point she learned her lesson and decided to just deal with the situation she was in without too much whining.
Happens once in All Quiet on the Western Front, when a newbie in the trenches is getting hysterical to the point of trying to leave the bomb shelter. Everybody else in the shelter beats him up until he doesn't try to leave any more. The narrator tells us that it's not pleasant, but it's the only thing that helps.
Nanny Ogg delivers a much-needed slap to Granny Weatherwax in Wyrd Sisters, less to calm her down and more to focus her rage. It's so powerful that it lifts both of them off their feet.
Used repeatedly in Making Money to interrupt Hubert Turvey's involuntary bouts of insane laughter. Igor slaps his cheek to do this, while the fastidious Mr. Bent pounds Hubert on the back.
In Wizard and Glass, volume four of The Dark Tower series, Cuthbert punches Roland to get him to focus on their mission instead of sneaking off with his girlfriend.
