This fic was actually was meant to be part of a larger story that I realize probably won't happen, so I just polished it off and posted it now. So please enjoy.
Disclaimer: The BBC owns Doctor Who. I am not the BBC; therefore, I do not own Doctor Who. I'm just borrowing some of the characters for a bit.
Mending a Broken Promise
There he stood, a gun in his hand, the fate of the world at his fingertips. He knew this was the end, not just of his life- ten lives to be exact- but of everything he stood for, hoped for, yearned for. He had spent the last few decades mourning the very people he had destroyed, was about to destroy again. Throughout the years, he had made many sacrifices, but he knew they were all for the greater good. Now, everything was on the line. It all came down to him. On one side stood his greatest enemy, and on the other stood Gallifrey's most powerful Time Lord. He had always counted on his mind to save him, but now he couldn't think above all their desperate, forceful words. He just needed a moment, though time itself was ripping at the very seams. This was a decision that he never wanted to make. He did it once before, and it left him empty and bitter. He didn't want to do it, he couldn't, wouldn't.
And then she lowered her hands. She stared right into his eyes. All of his broken promises, his distorted rationalizations, his empty vows: they all suddenly ganged up to mock the great savior of so many worlds, the destroyer of so many individuals. Katarina, Adric, Astrid, River, Donna... And now one more to add to his trail of pain and loss, only which to choose?
But her eyes... They looked at him with the same expectancy that they always had. Years ago, he forced himself to say goodbye to her. It was a hard decision, but he never for a moment thought it was the wrong one. He wished things had been a bit different, to be sure. He wished he could have seen her progress into the strong, brave, wonderful woman that she had become. The little girl whom he had raised within the confines of a broken down TARDIS had believed in him, had trusted him, more than anything in the universe, and the woman that stood before him now did not waver in her feelings for an instant. He knew he couldn't let her down... not again.
He would not mar her final memory of him with the stench of blood. No... he had one last trick up his sleeve.
A/N again: The Lady in White's identity is not canon. And it totally makes way more sense for her to be Susan than his mother, in my opinion, anyway.