I can't even count how many. But the ones that spring to mind are these. I had 2 playthroughs of FFXIII totalling over 150h but I could never get the plat because I couldn't beat the last mob. I never actually finished GOW3 because I got too frustrated in the fight against Zeus. I quit, watched a video of the fight and ending cutscene and called it a day. I burned out on ACO because it was just too big. I think I was about 70% through.

My most notable game that I could never even come close to finish is Kingdoms of Amalur. I played it on 3 different occasions over the years for about 50h each but I could never get past half of the game because I burned out before it. I really like the game but there is just too much loot and quests.

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