The following occurs at the end of Turn Left, in the brief cut between Donna hugging the Doctor and him sitting down to pick at the dead time beetle. I know it stretches continuity to put a scene in there, but I'm doing it anyway.
Disclaimer: Doctor Who belongs to the BBC. I just take him out for a spin now and then.
The Daleks could have died at his hands before they were ever born.
The creature fed off of change, of choices not made, of roads not taken. There was a choice he had made once, one of many regrets, but this one had earned a special place in his nightmares.
A sudden madness seized him. He grabbed Donna by the shoulders. "The fortune teller, Donna, what did she look like?" Donna shook her head. "What did she look like!" he shouted at her.
She flinched away from his intensity. "Asian, young, very pretty," she babbled, gesturing vaguely with her hands.
The Doctor exploded away from her, searching frantically through the fortune teller's stall, shouting, "Come out! Please! I just want to talk to you!" He upended boxes, threw fabric curtains about, even sprinted away to search the crowd in which too many people matched Donna's description.
His manic energy, driven by mad hope, spent itself. He found himself back outside the fortune teller's stall. Donna caught his arm and led him to where the time beetle lay. "I thought, if I could catch one of them," he said, "I made a choice, a long time ago. It cost my whole world. Countless worlds. For a moment I thought I might be able to unmake it."
Donna grimaced. "I don't think that thing would want to change the universe for the better."
The Doctor stuffed his hands into his pocketsand scuffed the floor, looking at his feet. "You're right, Donna. They never do. They create chaos and feed on it." He picked the beetle up and turned it on its back to poke at it with a probe. He supposed that the Trickster Brigade would never have allowed him to change that moment unless the resulting world would have been even worse.
Donna was talking, telling him that the parallel universe that had created itself around her was fading from memory, like a dream. She had asked him a question. He had best try to answer.