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Scattered across time and space. Everywhere and everywhen. She was so many people, she did so many things. Nothing in common but one single strand of fate - the strand that brought her here (and there, and everywhere). The strand called the Doctor, who wound himself throughout the universe more tightly than any one being had ever done (will ever do, was ever doing). His strand, attempting to be cut short but needed to remain whole. A universe without the Doctor wasn't this universe at all. And she knew that. She had seen his strand weave through creation (up and down and in and out) and knew that if there was anything she could do to keep that strand, she had to do it. So she jumped. She followed his thread back and around, repairing any damage she found. She saved him, so he could save her (had saved her, was saving her) and all the others he had saved (will save, was saving).
She knew that saving his strand would unravel her own. She knew the cost, as well as he did whenever he had to save someone. She also knew the cost of standing by, of letting him unravel. The people and civilizations and planets that he alone had helped stand instead of fall - all would disappear. She couldn't know, but almost did, that even if she didn't choose this, to jump, to fall, to shatter, the fates could cut her thread short, cut it before it even began. A leaf that never fell, an impossible girl never born, a Doctor never healing.
She finds herself alone. His strand repaired, her task complete. She's surrounded by nothing, just the remnants of his broken thread. She didn't expect this. That there could be an after, that she could possibly do any more with her life. She's scared, it's been so long since she's been this lost.
Then he's there. Her impossible Doctor. The final proof that his strand remained unbroken, that she won, that it was all worth it. Her impossible Doctor, saving her again, just as she had saved him.
The universe weaves a complicated web, constructed with many instrumental strands of life. And Clara Oswald is one of the most necessary strands of all.