He doesn't want to go.
He's dying. He can feel it. Even though it's happened so many times before, he's petrified. He can vaguely remember what it was like for the others, faintly feel the echoes of their personalities somewhere deep, deep inside. It's terrifying. He doesn't want to be left alone in the darkness. To cease to be. To have no more time.
He can feel the new personality, fully formed, just underneath the bubbling turmoil of his mind. Just like he can feel his hair itching to grow, his face to change structure. It's struggling slightly. It wants to exist.
He can't think. His mind is running at a thousand miles a minute, too fast, incoherent. Only one thought is slow enough to burn itself upon him.
I don't want to go.
It echoes and grows and leaks into every other part of his barely-functioning brain. The panic wells up, and before he knows it, the energy is escaping and the words have tumbled from his mouth.
The new personality is fighting now. Fighting to be free. To just be.
Even as he's fading, he still mentally screams and screams. And then, he's plunged into nothingness and he's nothing but an echo and he's fading.
And then there's another him, and he's trying to clear his mind from the weak, weak, weak remainder of... himself. And then the dying Doctor is no longer there. All there is is another man, another Doctor, stumbling away, hoping he's still got legs and that he's going to survive the crash.