As someone nearing 40 myself, I can relate.
Game stamina does decrease as you age. I used to play the same game for literally all my free hours, for weeks at a time. But my body can't handle sitting still for 10+ hours anymore; I simply fall asleep if I do that. I need to walk about a little, take breaks, make a decent dinner or whatever it is to break up the monotony. You can combat the headache by drinking ample amounts of water (have a water bottle available!), and replenishing your falling energy by eating (at least more often than every 10 hours). But it is certainly a stamina issue.
More interestingly, the ability to get
genuinely excited about something will dabber off. That's just part of getting older. You've seen this shit
before, you know. So what if it's a goddamned gorgeous game with eyecandy graphics aplenty - the gameplay is just like that previous game you played, and you already mastered that, so this game isn't really much new. XP bars and talent trees, with minor percentage damage upgrades? Yawn. Old news, not really exciting. It's too easy to lose interest quickly, probably even before finishing the game.
Combined it makes 10+ hour gaming sessions much more difficult.
BUT!
Every now and then, you find a game that is so genuinely good that you rekindle that spark. I guarantee you these games exist, the last for me was factorio and Mario Odyssey (despite not even really liking 3D platformers that much). These games are just very long inbetween. In addition, I still get genuinely enthralled in game
stories. And when I do, I almost always finish the game, even if my body is telling me that I'm kinda
done with this type of gameplay. My brain just says "no, this is too good. Keep going". Case in point, both Persona 5 and Trails of Cold Steel certifiably qualifies.
Treasure the games you enjoy. Forget off the ones you don't. There will always be something great out there. Look forward to them, because you will likely never grow out of them. Unless you want to