His body is dying, all he can do is slow the process down. He's a Time Lord who's running out of time, and the irony preoccupies him.
He spends more and more of his time in a healing coma, not only to restore damaged tissues and reduce stress on his body, but also for the escape it offers. Oblivion gives him relief from the pain of exposed nerves, and he craves it more than any drug.
Sometimes he imagines, just for a moment, not waking up. Embracing the oblivion that death offers. But he forces the thought away.