Maybe it really d-depends on how fun that game was for you 1 y-years ago. If it was really fun for you, then it could have d-drasmatically increased your average play time for each day, and your max energy. Worth mentioning is that the 'pounds' thing m-may have impacted you a lot. Weight may affect a lot your average energy and average play time.
Probably in this t-time it wasn't as fun as a y-year ago. Maybe you've done something important, like getting in h-high school, or if you've got a j-job (these can affect how much you can p-play, expect in Summer).
For some reason, I d-don't feel tired when I play a game, and nothing hurts when I play for a long t-time. Back in years ago, when I would play 2D games, I would feel fine, but w-when I tried 3D games for first times, I got slight nausea and I n-nearly vomited. This problem has since been adressed as playing 3D games now makes me smile, instead of harming me.
TLDR: Important f-factors that impacts your mental stamina are:
1. How much you play games (Longer game sessions tends to give you more stamina. I could probably last a whole 10 hours playing a 3D game)
2. How early did you get to play games (As a adult it may take alot getting used to it. Fortunately, I was a kid when I got a Nintendo DS, so it's all well...)
3. Your diet (Being lighter tends to give more mental stamina. I'm pretty light, so I think I have a lot of mental stamina)
These f-factor are all of what I know in my knowledge. If you wish to l-learn more, please go to Wikipedia and search the mental energy, or something like that.
I h-hope this has been helpful to you...