Misery Loves Company
He is exhausted; for hours he sat at a safe distance, watching Gallifrey burn until nothing was left but atoms drifting in space, and now he feels the effects of the war on this body. The thought of regenerating, of starting afresh, is a welcome one. He speeds up the process, something he has never done before. He probably could have just healed himself, but he never wants to see this body again; especially these hands. It is the closest he has ever come to regenerating on a whim; most Time Lords did it, but he has always been too busy to worry about which body he has. A vain bunch, those Time Lords. Silly and materialistic and so cross. All those names he's called them for all these years; all the epithets to describe them. Now he can add another, a last one: dead.
He focuses himself on the regeneration, fueling it with his anger and self-hatred, and is not surprised when he wakes up hours, or perhaps days, later with a tall, angular form and hard eyes. But he feels a bit better; all the very worst parts of his last regeneration are locked away for now.
He feels a change in the Tardis as well; she has changed a lot since he stole her away, but this time is different. She doesn't ride smoothly anymore. He experiments multiple times, materializing and dematerializing again and again, fiddling with settings and controls, and soon gets used to being flung around the console room. He finds the most uncomfortable places to hit, and the best places to hold onto.
Even after rebuilding her, she continues to shudder, start, rock back and forth. She acts like he feels, damaged beyond repair. And it's fine with him: misery loves company.