sticking to games I've tried from scratch a few times over but haven't beaten yet after years of effort...
X-Com UFO Defense [PC] - I keep getting stuck, either swarmed by aliens, go into dept, every nation signs up with the aliens, or most recently went too heavy with high tech crafts and ran out of special fuel for them.
Body Harvest [N64] - When I was younger I tried with cheats, then tried from fresh, then tried again, and again. I can't get past damn Java! There's I think 5 time periods in the game and I can't can't get past the second. I'll try again after I finish some of the games I'm playing now.
Luigi's Mansion [GameCube] - The first time I played it I got near the end, tried the last door too soon, had to go catch ghosts to unlock said door, went back when I had supposedly the right amount, and the door wouldn't open at all, I couldn't even interact with the door, I think the game broke on me. So I tried again a while later with a new save, and just lost interest near the end. Started another game, lost interest half way through. I love the concept, Luigi ghostbustering through a mansion, but at some point I just can't keep going.
Hybrid Heaven [N64] - I almost was at the end, almost there, had a lot of fun with the game, then it glitched, froze, and I lost my data. And since it's all its been doing, freezing, sometimes twenty minutes in, sometimes an hour, I was having fun body slamming bio-organic monstrosities too.
Legend of Zelda - any - to be fair I've only had three, but I can't get past them. I gave Orcarina of Time to someone, so there's no more trying that. Manjora's Mask I still have, but can't get very much done, I'll have to try again sometime. and Link's Awakening for Gameboy I was doing well in, then got stuck, went online for help, used a guide, beat a dungeon, then I didn't save, and two hours was for nothing.
then the rest of my gameboy games I haven't played much, but through a few hours of each I still haven't finished
Battletoads - hard
Super Mario Land - keeping losing interest
Super Mario Land 2 - to be fair haven't played it too much
Ghostbusters 2 - I like this game, possibly my second favorite Ghostbusters game. Anyone who doesn't know it, you play loosely through Ghostbusters 2 as any two ghostbusters, where you control one and the other follows you, where you move through each level looking for ghosts to zap with your proton gun, then while they are stunned use the second character to throw a trap to catch the ghost. It's fun, simple, but I suck at it.
side mention to Persona 4, I've been neglecting it, I was playing it too much and got drained. I think for almost a week I would wake up, play Persona 4, eat, play, go to sleep, wake up play, repeat. Through out the whole day. Then I was raking my brain and scribbling notes on how to fuse missing personas.