I just realized that Donna would have been feeling everything The Doctor felt when though she had his mind. She would have understood the love for Rose, the sadness for having to leave her, all of it.
She would have been sharing his suffering, and when The Doctor made her forget, he would have known that. He made her forget to protect her, knowing that he was wiping the mind of the only companion he’d ever had that understood completely his knowledge and his pain.
That must have killed him.
omg why does every whovian post here tear my heart in pieces?