Digimon World DS: I finally finished the factory dungeon and got the rank of Gold Tamer. However, in order to get out of there and beat the boss without grinding levels, I used a Digi-Volve Disk for the first time in my walk-through. However, by the time I got to the Sky dungeon (this is when every other level is closed for some reason), my team was severely under-leveled and I had no chance of beating the game legitimately. I restarted my game a few months ago, but I haven't been interested in playing it that much anymore.
The Legendary Starfy: I got to Mashtooth and managed to beat his first and second phase, however, his final phase where you have to press the Y Button several times in order to push back the moon in order to defeat him I could just not accomplish. Although I got all the collectibles, I wasn't able to finish the game and haven't tried battling Mashtooth ever again.
Kirby Planet Robobot: I had just beaten President Haltman and was about to face Stardream. However, I was not prepared for the control scheme that they were going to throw at me in order to fight it. Instead of a normal boss fight, it was a shooter, and you know how much I hate shooting games! I just couldn't come close to beating Stardream, and even if I could grind lives I still wouldn't beat it. Star Allies had a boss that was similar to this, but it was a lot easier and simpler to understand since there were some small sections where you played normally.
I also stopped playing Pikmin 3, NSMBU, NSLU, Mario+Rabbids Kingdom Battle, Pokémon Pearl, Heartgold, Black 2, Y, Battle Revolution, Pokepark Wii, Pokepark 2, Crash N. Sane Trilogy, Super Mario Maker, and Super Mario Run without beating the main game.
Reasons:
1. Got to the level where you were supposed to find Charlie and just quit the game after getting there.
2. Only got to World 4, than stopped playing.
3. Only got to World 2, than stopped playing.
4. Couldn't beat World 2-8 without using a guide, quit after trying the level a few times.
5. My mother stopped playing after beating Volkner and getting the final badge and I had no interest in challenging the Elite Four and Cynthia.
6. Beat the first 2 gyms, than stopped playing.
7. Same as 6.
8. I managed to beat Malva and Drasna and would go on to battle Siebold. Though I managed to beat his Clawitzer, the next Pokémon he sent out, a Gyrados, managed to crush my team. I stopped playing after that and haven't challenged Kalos' Elite Four again.
9. Got to Neon Coliseum, and just couldn't beat it because of that wheel you spin just before the battle starts. The Pokémon the computer players use are very lame, and if you get stuck with them instead your own Pokémon, you're in danger.
10. Finished the Meadow Zone and entered the meeting place, only to not be able to reach where Chikorita was heading. I jumped and jumped, but just couldn't get there and I wasn't willing to ask anyone what to do next.
11. Got to the lab in the Tech Area. After watching my older sister play the game and seeing that after you enter Wish Palace, Oshawott, Tepig, and Snivy lose their memory of Pikachu and you are forced to befriend them again just made me feel sad. I didn't want to go through that in my own walk-through and feel sad again, so I stopped playing.
12. Got to the 3rd warp room's boss after losing so many lives in the previous levels. Did not want to lose any more of them, so I stopped playing.
13. I just did not want to wait for new items and enemies to arrive in 24-hours, so I stopped playing.
14. I haven't committed to paying the $13.00 price tag to access the full game and probably never will.
I had a similar affair with Kirby Triple Deluxe where I just couldn't beat Sectionia in her true form and stopped playing until I started grinding lives and somehow getting to where you use Hypernova Kirby to battle her and I somehow managed to beat the game. I guess I got lucky that time!