For some reason as soon as I get to the "end" of the game, I completely lose motivation to play. This happened with DOOM: Eternal, Sekiro, Bloodborne, Paper Mario TTYD, Cassette Beasts, Shovel Knight, Oblivion, Baldur's Gate, God of War, Mass Effect... Pretty much every game I've ever played. It's like the fun part was the journey, and I've seen everything the game has to offer, and I just don't really enjoy the ending process. Feels like a formality at that point.
Sometimes, especially if it's a very difficult and long boss fight. I'll be less motivated because there isn't any gameplay after that, so I might just watch the ending on YouTube instead after a few unsuccessful attempts.