Sadly, this has kinda become my modus operandi as of the last few years. x)
Dark Souls 1 and 2 are the first games that come to mind. You have no idea how close I've come to finishing both of them, especially DS2. The thing is I've always played them co-op and though I've done multiple playthroughs of both like that, something always happens before I reach the finish line. I've literally done everything in DS1, DLC included, multiple times but for some reason we stop or they finish without me or other such things. I should have just pushed through and finished them anyway because I forget where I was and start over, the cycle repeating.
Fallout: New Vegas and Fallout 4. In both cases I go into a modding trance and before I know it I'm like 300 mods deep. With that much shit, something will inevitably break and I'll start over months later and repeat the cycle. x) I have some kind of problem with my unstoppable need to mod some games and this was before my brain started trying to kill itself! FO4 is kinda stinky anyway, so the prospect of going through the same stuff again kills any urge to finish it.
Darkest Dungeon. Same as above. There's just so many cool classes and new enemies and other stuff! I just can't resist. ;3; DD hasn't broken yet, I just get unsatisfied with some of the classes and want to start over after revising my mod list.
Front Mission 5. Such great gameplay, but the (lack of a) story and characters makes me eventually lose interest. The fact that a ton of cool parts and equipment are locked behind the Survival Simulator also contributes to this.
Digimon World. What a piece of shit that I am also in love with. I think most of my significant amount of hours in the game has been spent training or making a mad rush for a toilet. Fuck you, Digimon World! No, wait, come back...
Finally, Xenogears. My last time I was most of the way through the second disc, but we modded my PS3 and I forgot to backup the save file. Fuck. I've owned a copy of the game for like 25 years and I've started it over many times, but it remains unbeaten. I've spoiled myself on everything, but I still want to experience it myself. I just absolutely love the world and the backstory.