Something is coming—he knows it, with every blink and heartbeat he can feel it. His time has always been unknowable, shorter than any other of his people, but now he can feel it slipping through his hands like melting snow.
Alone again: surely that is part of it. Rose in the parallel universe; Martha working for UNIT; and Donna—oh, Donna Noble, most important woman in all creation. Her time with him gone, wiped away like a child's nightmare.
Now he is the nightmare, a dream aware of the waking sleeper, and trying to suppress the dreamer's consciousness. He feels a taste of John Smith's terror, the desire to stay normal as unalterable doom approaches, betrayed by one's own mind.
When it happens—when he knocks four times—he'll die. He may have downplayed regeneration's significance in the past, but now he can't lie to himself anymore.
He'll die.
Some new man will wake up. This new man will stretch, roll over, and dress as he sees fit. The trench coat, suit, and Converse will be shoved to the back of the closet to make room for a new outfit. The new man will run out of the TARDIS, gripping a sonic screwdriver, and finding a new friend to travel the universe with.
And he'll be forgotten, like a drab dream when the sleeper wakes into a busy, exciting day.