She was there with him, watching him try and fail, and try and fail again. And she couldn't stand by and let him continue, trying and failing, trying and failing until it was too late.
To him, she was nobody, a small nobody from a faraway planet, whom he happened to bump into. That was all, she thought. He was a clever, handsome man with no need for someone as silly as her. He would never look at her. But she was happy to follow him, loving him silently and eternally. When he needed help she would be there, offering everything and anything if it meant she could stay with him. Already she felt her world would break if they parted,
When he asked her to come with him, she felt she might burn up with the happiness that filled her inside. She hesitated, perhaps too long, though for a Time Lord, she doubted he noticed. And for a while, she was filled with a certain glow, the glow that comes from inner content, content that can't be put out. Sometimes she doubted he meant it, whether he would keep the promises he made, but then something happened, some danger or feeling or happy thing, and she didn't care, her worries swept away by the feeling of living in the moment and letting the water flow on by.
He had left her, if only temporarily, but she hadn't known that at the time. He had disappeared, let the enemy carry him away. She had thought perhaps he didn't like her, that this hurt she felt was manipulated and manufactured by him, but she knew in her heart that was not so. He left because the universe needed him, and she wasn't quite his universe, yet. Some things are bigger than infatuation and love. Some things require sacrifices, sacrifices we dread to think about, but this was what she was prepared to do for the alien man who had changed her life.
She couldn't let him fail, the universe needed him, she had learnt that much. So she did what she could, destroying the enemy and leaving his path clear to save the day, no matter the cost to her. And she was on fire, falling, falling, burning, burning, and then there was blackness, and she failed to remember.
But then she awoke, awoke far from where she had fallen, her memory damaged but her mind intact. And then she wasn't falling, she was flying, flying, and the atoms of Astrid Peth roamed the universe, forever with her Doctor.