Prologue:
Ancient Greeks believed no one could escape their Fate. Greek tragedies were written under the concept that running away from Destiny would make it come true. Several stories and myths repeated the same fundamental belief that what is meant to be will inevitably happen.
The Doctor always refused to believe in Fate. With a few rare exceptions, Time could always be rewritten. He had seen it happening, over and over again, but, every once in a while there were moments in time that couldn't be changed, no matter how much you wanted to, no matter how much you believed that it would be for the best, they would still happen. Those were fixed points in time and if anyone attempted to change them, Reality would collapse and Time would disintegrate. But those fixed points were very few and rare.
And sometimes, every once in a while, when you attempt to change Fate, you end up by making it happen. It was a painful lesson he learned by experience that the most benevolent interference in History could lead to terrible results. A simple trip to Rome could lead someone to Pompey and just by being there and wanting to prevent a terrible catastrophe, you might be forced to cause it instead. And then, just then, in that precise moment you go from being a simple witness to becoming a mass murderer, causing the same tragedy you wanted to prevent.
So much blood on his hands and so many painful memories finally made the Doctor realize that Ancient Greece knew something about Fate after all. But even if he did know that, he would never submit to it. Time would never be the boss of him, and he had to believe that when it matters, it could be rewritten, and rules be damned. He was the last of his kind: the last Timelord in existence and he would never accept to be Fate's victim. He would struggle with all his strength to change whatever events brought them here, at this Library. He would do everything in his power to prevent this moment to happen. And it should be easy, because he wouldn't even have to visit the past. He would just have to change his future.
"Time can be rewritten" he cried desperately to the woman in front of him.
"Not those times. Not one line. Don't you dare!"
He would dare, because he simply couldn't ever accept this. Here in front of him, minutes ago, stood an amazing woman that sacrificed her own life to save him, an old man who had lived too much already, who had seen too much already and suffered too much already. Maybe it was past his time to die, and this woman sacrificed it all because she loved him. She did it because in the future they would be together, and she loved their story, and she loved their future. She did it for him and for them. But if she never became close to him and if she never fell in love with him, she would have a chance. He just had to be sure that this future never happens, and she would be alive and happy in the future, as she deserved to be.