All veterans here having the exact same experiences hehe... I am on the same boat, even though I still enjoy the drugs from time to time, the usage hiatus are bigger and bigger, and sometimes I easily talk myself out of some binge because I know the price I will have to pay the next day or days...
I think it's just the body getting older and not regenerating like Wolverine when you were on your 20s, like people mentioned here, at that time you could go out every night if you wanted to, get wasted on booze, coke, pills, whatever, get home 8,10am and sleep for a couple of hours and wake up 100% rested and ready for the next party. Shit... Even if I drink 6 cans of beers nowadays it already gives me a hangover, I am 41.
I used to be a classic stoner from 15 to 19yo or so, then I dropped it, it became really bad, being stoned just made me paranoid and feeling like shit, never smoked again after that, I think that the only occasion where some puffs will make me feel good is when MDMA is starting to comedown from the peak. Other than that, I even hate the smell of weed.
I used to use psychedelics heavy, mainly shrooms then ketamine, then just grew out of them, don't have the willpower to endure trips to other dimensions anymore.
I used to love MDMA, but nowadays I just get underwhelming rolls and very very nasty comedowns, specially the day after I will be total trash, zombie state, depressed, mouth all sore... horrible. I probably felt the same thing when I was younger but just didn't care or I was able to endure it easily.
The only drug that still hits me like always did is coke, but it also has it's toll... Fucks up all your nose, I get a clogged puffy nose for weeks. And if I don't stop doing it early I won't sleep, which will ruin my next day.
In a nutshell I think that the fact of the older body not regenerating as fast as the younger body (worst and worst and longer hangovers), and the fact that all of us here abused drugs several times way more than a "normal person" would (Crazy nights, crazy benders, etc...) meaning "been there, done all that already lots of times" contribute to "not having the stamina for drugs" anymore.
I think I have partied enough for 3 lifetimes, seriously... I wasted my whole 20s with only parties and drugs, and I Really regret it now having wasted a decade of my life not going anywhere. I wish I were where I am today, but 10 years ago...