I think I was doing poor because of my team set up. I had Lightning, Sahz, and I think Snow. I refused to change the team since I didn't like the other characters, (except Fang, she's fine). I kept losing, and I couldn't stand it anymore, so I quit.

I do like Sahz but he's the weakest character (in terms of damage) of the bunch. Snow is alright if used well but I never needed a Sentinel, so I barely used him.
AFAIR it's really important to have a Saboteur and a Synergist in the party.

With Hope, Vanille and Lightning it's terribly easy. All 3 are Medics, all 3 are Ravagers, Lightning is a Commando, Hope is a Synergist, Vanille a Saboteur.

Metal gear rising beat me, fighting against monsun again wtf

Yeah, my favorite character was Sahz, so I was determined to use him as much as possible, while I absolutely loathed Hope, which is why I refused to use him. I learned those characters were the best, but I strongly disliked them, so I couldn't beat it in the end.

Demons souls. No explanation needed. I didnt even passed the tower knight.

Many games, most of them recently. I simply don't have the patience to endure games I don't enjoy for whatever reasons.
This is just a selection of games left unfinished:
- Lords of the Fallen --- I don't find enjoyment in repeating checkpoints a dozen times. Enemies are very tough, the game's pacing is painfully slow, it takes minutes to defeat the simplest opponent. And what I found most irritating is enemies respawning as soon as you walk behind a corner. Kills any will for exploration. Endured the suffering until I beat the second boss, then quit forever.

Shadow of Mordor - just too much extra stuff to do. I can hardly ever make it to the mission, as there is always some enemy to fight, some captain to defeat, somebody to avenge... It doesn't help that everywhere I go I need to either fight three orcs that multiplies to thirty in a heartbeat, or that if I want to avoid them I need to sneak around like an idiot. Makes even the easiest collectibles a chore to get to. Sure, there is lots to do, but it just seems to go nowhere.

Alien Isolation --- 4 chapters of sneaking through vents is plenty enough to call it quits, when I kniw there are a dizen more to go, with androids around every bend. A very slow and boring game.

Bound by Flame --- well, this game just plain sucks.

Bioshock --- once I realized that the entire game will be me shooting stuff in a submerged corridor, I gave up. That was chapter 2.

Honorable mention: The Last of Us --- I actually finished this game. I persevered because of a promise of things getting better, sadly they never did. I started on PS3, made it about halfway until I just couldn't take it anymore. Then I borrowed the PS4 version, had to suffer through the same part ("It gets better right after that part, trust me" said some of my friends). I vowed to finish it, never had I been praying more that each chapter is the last one. About 5 chapters after I had enough, the game mercifully ends.

There are more games I never finished, these are just a few recently. 

Sure, many times actually. But mostly because I have too many games and not enough time. I estimate I have about 75 games divided among my consoles which I haven't even started played yet (I know it's ridiculous but I keep buying cheaply if I know they're good, even though I'll never play half of them). If a game is not as good as I hoped I just stop playing it.

So I'll just focuss on games I didn't complete because of difficulty. Happens a lot with SNES games, many of those games are inceredibly difficult.

Modern games tend to be a bit more forgiving so I generally complete them if I like them (though rarely 100%). Dark Souls was difficult but I managed to complete it. Demon Souls is supposedly harder though?

More recently, I haven't completed Champions Road in Mario 3D World or the Perfect Run in SMG 2 yet. They are absolutely insane.

I also completed all levels in DKCTF, which is pretty difficult, but I decided not to try and complete them all on the unlockable Hard mode. 1 heart and no checkpoints is absolutely brutal.

24 hour endurance races without a save feature during pitstops.

If you can't play for 24 hours straight (well, you can pause to sleep, eat or go to the toilet but you can't turn the console off) for whatever reason like me, then the only option is to have the game play by itself because when the IA drives your car you can speed things up by 3x (still over 8 hours). However the game resets to normal speed after each pitstop (and wastes precious seconds if you don't choose the new tire compound quickly) so you have to keep an eye on it the entire race.

I have better things to do with my free time and my PS2 than that so the file stays that way.

Marvel vs Capcom 3 - 1 trophy away from platinum but I cant/not dedicated enough to do all the required combos challenges (I did about 60% of the required amount)

Street Fighter 4 - similar situation as the MvC3 doing combo challenges with just a few trophies away from platinum.

....Of all the Capcom DLC i wish there were DLC where we can purchace the game trophies loll I hate it that im so close to platinum but can't lol

Quite a few old games, mostly NES & Genesis. Edit: I see most on here seem to think not 100%ing a game is defeat...