Quite early on, Rose Tyler realized that she had to decide whether all of it was real, or none of it mattered. Thinking that her universe, the universe with Her Doctor in it, was the real universe had been a trap. That had been bad.
A bit into it, after seeing them all die, and die, and die, reality slipped sideways. Unstuck in time, she flailed at the horror in her mind, struck out at it. It was like punching thick fog. It was choking and untouchable. And it offered absolution.
When it lifted, she was not quite who she once was.