The moment of realisation fell upon Donna. She wouldn't die, no, Rose meant this universe would die!
"'Cos I understand now. You said I was gonna die, but, you mean this whole world is gonna blink out of existence." She barely noticed the sad look on Rose's face, and continued on. "But that's not dying - cos a better world takes it's place. The Doctor's world. And I'm still alive." Her breaths got shorter and the smile vanished from her face as she noticed Rose's face was not one of someone in triumph, happy for her to have worked it out. It was of sadness, regret. "That's right," she said to Rose, asked her. "Isn't it, I die, if I change things, I don't die, that's, that's right, isn't it?" Suddenly Donna realised she was very wrong as the blond-haired woman said two words that made her realise her life was coming to an end.
"I'm sorry."