Some of the numerous games that I have quit and I can remember:
Final Fantasy VIII: Stuck at the very last boss fight with Ultimecia.
Tomb Raider III: I have tried it a hundred times, but I always run out of crystal saves in the Nevada levels. Too damn difficult game with difficult jumps.
Resident Evil Remake: Hard as hell, with the zombies that revived after a few hours and were twice as hostile than before. I tried it twice then I decided to play the original game instead.
Final Fantasy XII: I was at 2/3 of the game, but I was so fed up with the unreasonable battle system, the confusing, pseudo-epic story and the androgyny characters that I quit.
Eternal Sonata: The cure for isomnia. Well..I got cured!!
Apache Air Assault. All were going fine in this very enjoyable helicopter sim, until the notorious Water Sharks mission, which is simply impossible for an average gamer.
No More Heroes: Heroes' Paradise: I was trying to be patient with this annoying game but after the fourth assasination mission, I got totally sick of its immature style, the primitive graphics, the nervous controls and the environment glitches, you know, those that you get stuck into, and have to reset the game.