Pride and Prejudice: Elizabeth Bennet, in a way, fits this. According to her, assuming the worst-case-scenario of first impressions would protect her from being duped about people's true nature as bound to happen to her all-time Wide-Eyed Idealist sister Jane. When she learns the truth about Mr. Darcy, she realizes her method is just as flawed, having blinded her to the truth and let her be taken in by Wickham because she was so eager to believe his terrible story about Mr. Darcy. Wickham essentially took her in by flattering this aspect of her character that way.
Elinor Dashwood of Sense and Sensibility; her mother and sister Marianne are constantly annoyed with her pessimism and determination always to doubt the best-case scenario. Of course, she's usually right.
A Christmas Carol: Ebenezer Scrooge, contrary to everyone else in Dickens' Victorian London, thinks that Christmas with its wasteful giving to the poor is one big (you guessed it) Humbug! It takes a Jacob Marley Warning and visits from three ghosts for him to finally soften up and celebrate Christmas with everybody else.
Benjamin the donkey in Animal Farm by George Orwell. After the pigs have led the farm animals to chase away Farmer Jones, and are creating their own Utopia, Benjamin still exclaims, "Life will go on as it has always gone on – that is, badly."
Used in a brief segment of C.S. Lewis'The Last Battle. The last shreds of La Résistance are trapped in a stable. A faction of dwarfs that had split off from the main characters is also there. Due to the nature of things, the inside of the stable is actually a good deal nicer, but to the renegade dwarfs, it's mostly just a stable. They are unreserved in voicing their discontent with the situation.
Puddleglum comes off as this in The Silver Chair. Jill and Eustace eventually find out he's not. Even the way he comes off is seen as scarily cheerful by the rest of his race, at least according to him.
Subverted in which the universe he lives in is pretty cynical (it's Douglas Adams after all), but his depression is still disproportionate.
Eeyore of Winnie-the-Pooh is the second-most iconic Grumpy Bear. Surrounded by beautiful nature, blessed with happy friends and a loving human playmate/owner, Eeyore insists on seeing the world glass all-empty.
Also proven in the comic.
Eeyore: (To Pooh and Piglet) Where are you boys headed?
Pooh: Oh, nowhere.
Eeyore: (Looks back at sign on tree behind him, which reads "Eeyore's Gloomy Place") You've arrived.
Eeyore: Everyone around here is always smiling. I wonder what I'm missing.
Eeyore: Now I'll probably get laugh wrinkles.
And you can't forget:
Eeyore: Uh-oh, here it comes again.
Eeyore: I forgot to take my gloom pill this morning.
Eeyore is so pessimistic it goes all the way out the other side and circles back around to narcissism. He once assumed that a party was for him, because he couldn't think of a reason why someone might invite him to a party in the first place unless he was the guest of honor.