Rose haunts her. Every time she turns a corner, or opens a door, she expects to see Rose. Rose, who belongs here, Rose who had her place.
Rose whom he loved.
Everyday she decides to tell him to go back, to find Rose, to take her home. But her life with him is too exciting, the moments she shares with him too sweet, and she finds she cannot let go.
And ever since the day he told her he had two hearts, things have changed. She didn't understand his meaning. Two physical hearts didn't mean the same as having two hearts to give, much as she wanted it to. But she hugs that one moment to herself at night, in her bed, just before she falls asleep, as her first spark of hope.
Because now he takes her hand, sometimes, unexpectedly, when they walk together. He puts his arms round her as she stands at the console. Sometimes she catches him watching her with a half-bemused expression on his face, as if he can't believe she's there. And yesterday, when he thought she was dead, his hand shook and his face went pale, and he had almost killed the Sontaran he thought had killed her. And today he told her she was 'quite special. Unique, in fact.'.
Little moments like that keep her going, keep her warm. Something to remember at the end of the day, when she thinks she's hears Rose's voice answer his call, a still living ghost from the past.
But ghosts fade, and she promised to stay always. And the Doctor has two hearts.