You don't quite realise what you've done until you hears the doors closing behind you, because every other time you knew that they were going to open again. This time you're practically certain that they won't.
You don't quite think about what you've done as you set the co-ordinates. You focus on the numbers, the mathematics, because these are the things that you understand best; the curve of the console, the pattern of lights, because these are the things you are most familiar with.
You don't quite panic as the central rotor begins to rise and your TARDIS begins its final flight. It was the only option. It was so easy to choose because there was nothing else to do.
You can see that slight tremble of your hands, feel the slightly uneven beat of your hearts and there's something eating you alive inside as your mind struggles against its own refusal to make your thoughts coherent.
This, you realise, is fear.
Not the easy comfortable fear that has been your companion for centuries, but something so much more raw, more primal. And you think of your companions.
"How could you have done this?" you mutter with a shake of your head and a whole new respect for humanity. You realise how exquisitely naive you have been, because you never went out into an alien universe. You studied it for decades; a student of the universe, but you were above it, apart. You were superior and you knew it.
When you left Gallifrey for the very first time you thought you had finally found your freedom. For centuries you had the whole of time and space to explore and you revelled in your adventures, because you were so much better than your peers. You didn't want to merely study; you wanted to understand, to explore, to feel. But you already knew what it was like; you just got to have a closer look. Fear, excitement, expectation became intermingled and soon you couldn't tell the difference as each new world gave you a new reason to extinguish any of your lingering doubts.
Fuelled by the righteous anger of someone who knows that they know best you forgot that you often found yourself in mortal peril. You saw death in a thousand incarnations, and when it finally came for you, you embraced it as a new experience and forgot that you were only mortal too.
You always knew how to win because you knew the universe. You played different sorts of games across the centuries. You observed, you interfered, you manipulated. You saw how fragile other species could be and you found how deeply you could feel and how much you could lose when you grew bored of playing the universe's games and decided to design your own.
But you were never afraid, never truly paralysed by the terror of the unknown because this was your existence, your reality and you understood it.
You never quite comprehended why eventually everyone would leave you because you never appreciated that they couldn't call the universe their home. Yet there was always on more person willing to leave everything they had ever known behind and go out into the unknown with no one but you between them and every single terror that they never knew existed.
And you never knew what they did because ignorance had been beyond your experience for centuries.
But now you think of every man and woman you ever knew, every badly played tune, every perfect piece of art, every mathematical principle, every logical argument, every pointless disagreement, every scream, every laugh, every tear. Everyone and everything.
And you leave them all behind, as you finally face the unknown.