The Doctor is alone.
He feels it, an all-too familiar heaviness that settles in his hearts as he shuts the blue doors behind him. The nothingness of the air surrounding his body presses in, until finally he's suffocating on the emptiness and the silence, all around him, drowning him. There are echoes in the silence, filled with the promises of new adventures, laughter and friendship, just taunting memories now that he can't bear to relive or even think about without experiencing a deep, familiar pain.
They're all gone; Rose to another Universe, Martha to her broken family, Donna to her normal, oblivious life, Amy and Rory to their deaths, and even River to the Library. Gone, all of them, all shattered and broken in some way because of him. It's days like these he wishes he'd never started running.
Now he walks slowly, all 900-and-something years showing, up to the centre console, feet shuffling like a child. He then finds himself staring wistfully at the de-materialiser that's disguised as a lever, though not quite seeing it, as his face twists into an sorrow-filled mask. What he sees instead is Rose's heart-broken face, Martha's stony determination, Donna's tears and all three Pond's broken bodies. Before he knows what he's doing, he's reached out to strike, slamming his hands down on the hard console, letting his frustration out with three more hand-breaking blows that sends vibrations through the entire ship.
By the time he's done, he realises two things; one is that he is crying, a stupid, human thing that he shouldn't be allowed to do and two is that it's time to go, to leave, and never return. He wipes away the wetness on his cheeks with the cuff of his tweed jacket and sets to work, walking solemnly around the circumference of the console, working the familiar motions. He doesn't care for the destination, really. All that matters is that he gets there and that he's away from this place. He takes a deep, consolidating breath, before, with a whispered, "Geronimo," he throws down the final lever and is off once again, running through time and space, not once daring to once look back because he can't.
…
Because no matter what happens, the Doctor will always end up alone. :(