Had this same thing with Portrait of Ruin. As well as FFVII. Got to the final crater with Sephiroth and was under-levelled, every fight with a beasty on the way down would wipe me out. Didn’t want to grind and had enjoyed what I had played. Just left it, never to get that far again.
Back when I first got it, I had to take a break from the last level of Tomb Raider 1 as I just couldn’t do it. Left it for a few months, went back and did it in one go. Could not work out why it seemed so hard before.
On a non-game related unfinish, the first time I read LOTR I finished after Frodo destroyed the ring as I had recently watched the movies and came to the conclusion that the rest would be a medal ceremony from the Princess with snazzy one liners.
Read it again last year, continued past that bit and thoroughly enjoyed it. Incidentally, I also enjoyed the whole thing a lot more the second time than I did when I was younger (and I assume less intelligent)