Never.
Never say never-ever.
She can save a man from death by swinging on a rope. She can save a Dalek from itself. She can break open time and space with a chain and an old truck. She can defeat demons and metal men.
She can defeat anything – anything except the reality she herself creates. She can start with nothing, make a path through the universe, and end up with nothing once again, except for one thing:
He believes in her, and, more importantly, so does she, now.
She will never, never-ever give up, and she will never, ever quit.