In his dreams after the Year That Never Was, the Doctor remembers losing everything that makes him him over and over. He remembers learning how to speak again, how to walk and eat and so many other little things that you never realize matter until you've lost them.
He remembers the initial terror each loss brings, with the burning in his head and the harsh grip and cold, cold eyes filled with so many emotions he can't remember or understand. But he can sense the malice in them and so he wants to run from them, every time. He also remembers another presence that's always there, burning like a sun. It's always startling at first, a shock of time, age, presence and it is wrongwrongwrong, but it offers a way out of the cold presence of the other. It offers warmth and safety and a haven until he can remember again, its mind a comfort and a balm to the rattled chaos in his head.
There is a choice between the two that isn't a choice at all. It is offered over and over, until he thinks his mind might shatter even more from the forgettingrememberingforgetting. It is simpler to choose every time, with vague wisps of before always floating in his head. He always strives for the warmth, a refuge from the black maw of the other that is always trying to pull him in. It is always so, so angry when he reaches for the sun. But it always leaves them for a while and he revels in the ever briefer moments where he can rest, recouperate and just be. There is a simplicity in the loss and the learning after. But soon enough the other comes back and he remembers again, but loses the warmth and the simple comfort of that love and friendship, loses the memory of those times.
When his dreams come to that point, the Doctor wakes up crying. He can't remember why.