He wasted away watching “reality” T.V. shows.
Except that, in the context of his personal experience, he was never the most important person the world, any more than any of us are. He always thought he was just “some guy.” He only found out the world was watching at the very last minute, not nearly long enough to have even sunk in psychologically.
In fact, his point of view was just the opposite, he went from being just some guy to being world famous in an instant. THAT could have jacked him up, of course.
First he becomes really stupid, then really smart and funny, then he develops an affiliation with pets, then goes crazy over a number, then develops multiple personalties, then multiple personalities & maniacal & huge and green, then he hates christmas, then evil & green & steals christmas, then lies about everything, then cant lie, then forgets everything, then becomes poor, then comes out, then becomes the chosen one again.
Confronts Cristoff and Cristoff reveals
“TRUMAN - I AM YOURRRRRRRRRRRR FATHER!”
Whereupon Truman shoots him dead, gets a trial, goes to jail - and currently has a new show in his jail cell.