isn't he a clone of the real Solid Snake that deteriorates faster than a normal human being?
you're off but close enough
SPOILERSSSS
Solid Snake is a clone of Big Boss AKA Naked Snake, he was a legendary soldier and the Patriots wanted to clone him to create new soldiers to represent the image of the Patriots. He is a clone of Big Boss but in order to keep the clones from reproducing, or becoming a weapon of another corporation, they were intentionally made so they could not reproduce and with a shortened lifespan
ALSO why his life is depressing:
he loving does everything everyone tells him to do as a soldier as his duty literally used as a weapon rather than a person
his love life fell apart, his only true love fell for another guy
He had to kill his own father and brother
everyone is dependent on him to save the world
shortened lifespan
he has a virus in him that is mutating to make him become a WMD, he is saving the world from destruction when he is a threat himself
His mom dies because of him
He is forced to kill himself
He gets no break and endures so much loving pain it's depressing
and in the end he accepts it all because he knows that he is just a weapon. a weapon for war