The boredom is the worst thing.
Stuck inside his TARDIS, outside of time and space with nowhere to go and nothing to do is driving him bonkers. But he has no choice; he has been banished to limbo in order to save the universe. Because of his sacrifice, he is now a non-entity, never having existed except in this no-man's land where time and space are meaningless. He is now truly lost, having been forgotten by everyone, including the people who had become his family over the past few months. As he sits in his TARDIS that is drifting aimlessly though nothingness, his thoughts turn towards his former companions. Rory, the miracle man who was resurrected thanks to the Nestene Consciousness and Amy Pond, The Girl Who Waited. He sighs, knowing that she'll no longer be waiting for him in the past, that's all been erased now. He hopes that she and Rory are married and have a happy life, perhaps having children, children with red hair, the color of fire like she has. He chuckles, recalling her comment in the museum about the ultimate ginger. His Amy, free spirited, so full of fire and spunk, he hopes Rory will be able to live with her and stay sane.
As he sits by his console, his thoughts drift past Amy to his former companions and his hearts ache when he thinks of Rose. Since he was erased from time, was Rose back working at Henricks now? Was Martha working at a hospital, Jack still being a con man? Donna…considering what happened to Donna, not much would have changed for her but he still wondered if she would have gone ahead and tried to marry Lance. He grimaced wondering what the Earth would be like now without his protection. He shuddered to think of the devastation, especially considering how many civilizations had tried to invade Earth since it was first created. He shook his head, he couldn't think of that now. He'd go insane thinking of that without having any way to get out of the predicament he was in.
He got up from his seat and walked over to his console. He put his hand against the central column.
"Just you and me now, old girl," he said as the TARDIS rumbled soothingly above him. "We officially don't exist now. We're just two nobodies drifting through the void. Guess we'll have all eternity together now, won't we?"
The TARDIS rumbled again and nudged his mind affectionately while the Doctor walked around his console to the front door. He paused in front of it and put his hand against the wood, feeling how cold it was.
"Wish I could go outside," he said as he kept his hand there. "Wish I'd died when that Dalek shot me. Dying would be preferable to this. This is a living death. Maybe it would be better to just open the door, step out and float away until I died."
He shook his head.
"I can't do that though because I told Amy to remember me and I have to believe that she will. That brilliant, special little girl of mine, she has to remember me and bring me back from this hell that we're in. Because if she doesn't…"
"I REMEMBER YOU, I REMEMBER!"
The Doctor jerked his head up when he heard Amy's voice echoing overhead. His breath caught in his throat, hardly daring to believe as he listened to his beloved companion's words.
"I brought the others back, I can bring you home too!"
A grin broke out over the Doctor's face.
"Raggedy Man, I remember you and YOU ARE LATE FOR MY WEDDING!"
"Well then, if I'm going to a wedding, I better look the part. Right, old girl?" the Doctor said.
He turned and sprinted to the back door while the TARDIS moved the door to his wardrobe room so he could change as quickly as possible.
As he ran, he heard the TARDIS's rotor start up and he knew he was going home.
"I never believed in fairy tales before," he said as he hit the stairs that led up to the second level where he kept his clothes. "But thanks to you, Amy Pond, I'm a true believer now."