If you asked the Doctor when the flashback started, he'd say it was when everything turned silent. It was like he was blind, except he could still see Davros looking expectantly, waiting for emotions he could use to mock, or criticize. His hearts began beating twice as fast when the image of Jabe appeared. He felt a lump of guilt in his throat. He tried telling himself it wasn't his fault, but he did choose to go through the vent system alone, rather than somehow allowing for Jabe to go with him, keeping her safe.
It was then he realized what was coming. A storm of emotion. And he couldn't let it show. It'd be difficult, but the last thing he wanted was for anyone to see him cry. He was pretty good at avoiding it back on Gallifrey. He'd just run to the barn and cry there. The only one he ever had to face, was the Master, and that was just so the Master could comfort him. He took a deep breath and prepared himself.
The next one to truly hit him was the Face of Boe. He felt the tears coming, he couldn't stop all of them, but he wasn't at the breaking point yet.
That is, until he saw Astrid Peth. He had to close his mouth to keep himself from sobbing out loud. Then came, the big kahuna. River Song, the only woman or person at all, to know his name. He didn't know what he would be to her later, but certainly something immensely important. His whole face shook with the emotions he was keeping out. He looked seriously and darkly at Davros, keeping his eyes on his eyes, not the rest of his wobbly face. When Davros accused him of being ashamed, his mood switched partially, to blinded rage. He wanted to punch Davros to death for his remark. Instead, he simply took a deep breath, swallowed the lump in his throat and waited for what would happen next. The second Davros looked at Dalek Caan, the Doctor closed his eyes and imagined a place of calm, blank, pitch black space, full of stars. The last thing he saw on his last night on Gallifrey, and the thing he ran to all those years ago. The noise and silence met. He was alright. The Doctor was always alright. He was better than alright. He was fine.