He knows she's drowning.
The flippant, desperate denial, the urgency. The fear of the inevitable, buried so he doesn't have to feel... only think.
She knows. The thing that's killing her is the thing that's letting her know.
“Don't make me go back....”
He looks at her, and he hates his pity.
“Please... I don't want to go back.”
She remembers who she was, and it breaks his hearts to see her realise where this is heading.
She's drowning in him (don't they all drown in him eventually?) scrabbling towards the surface like an unwanted, but faithful dog.
She doesn't care he's broken her. She only sees who she's become and the universe- for a moment, glimpsed in it's full glory.
“Doctor, please. Don't make me go back.”
He can't see a choice. In a nanosecond he remembers how it feels to hold a slowly cooling body in his arms and he can't bear that pain again.
Damn it Donna, don't do this to me. I won't let you. I won't!
Because he's left his ghosts behind on a parallel world, and he wants no more created today. Because she's only human, she hasn't the will to resist him. Because he's a Time Lord and he has the right to change time. Because History can be rewritten.
Because she'd be better off having never met him, living her simple human life.
And not being dead. Not being dead is good. Good for both of them.
He can see himself dying within and beside her. So he strips away her identity, rips himself out of her. Saves them both.
It's only later, when he's alone, when he's finished cradling her on the doorstep of her house, when he's left her with her mother, that he finally realises what he's done.
History can be rewritten.
Don't you dare... not one line.