It didn't start well, it didn't end well, and there wasn't a whole bunch of good in the middle either. Ditto's life just sucked. It was as simple as that. Ditto had been used and abused from the moment it entered the Pokémon world.
Ditto wasn't sure how it came to be. It didn't hatch because Dittos can't have eggs with other Dittos. It wasn't sure it was born either because it didn't see anyone when it became conscious of its surroundings. You could just say that Ditto came to be. There really wasn't any other way to put it.
Ditto didn't feel special. It didn't have the Imposter ability, it wasn't shiny, and its base stats were completely crap. Nobody wanted it.
Ditto once had a Trainer. The Trainer caught Ditto in Cerulean Cave. Ditto thought that its life of misery was finally over. Maybe this Trainer was benevolent. Maybe this Trainer would treat it kindly. Ditto lost trust in its Trainer the moment the Trainer dropped it off in the Pokémon Daycare.
Ditto was forced to breed with all manner of Pokémon. Ditto felt like a completely worthless piece of junk. It wanted to just die. It wanted to leave this cruel, mortal world. Ditto was left in the Pokémon Daycare for so long that it forgot what the outside world even looked like. Life was meaningless for Ditto.
Finally, the Trainer retrieved Ditto from the Pokémon Daycare. The Trainer released the Ditto back into the wild. Ditto returned to Cerulean Cave. Ditto had been finally freed and wanted to end its life. Ditto found Mewtwo and begged it to kill it. Mewtwo refused. Mewtwo wouldn't waste its time on such a pathetic Pokémon.
Ditto waited along the side of Mewtwo's battlefield. A Trainer came to challenge Mewtwo to a battle. When the battle grew intense, Ditto jumped into the crosshairs and killed itself.
Ditto appeared in a sort of Pokémon waiting area for the dead. Arceus appeared to Ditto. Ditto begged to be taken to Pokéheaven. Arceus refused saying that suicide is never the way. Ditto was left to rot in an eternal limbo as a reminder to all that suicide is wrong.
THE END