If there was one thing he ever regretted it was leaving her. Leaving her, so many times, not even looking back. He did it for her, mostly, or he didn't know what was going to happen, or it broke his hearts, but he still left her. And look where that's left him.
All alone.
First when he made her go back to Earth, and he was going to destroy the Daleks and the Gamestation and himself. He knew he wasn't going to let her risk her life. If was for the best, as these things often are, but that didn't stop the pain.
Then later on, with the Wire, on the streets of London. Her face had gotten sucked off then, and he can still remember the burning shock, the overwhelming sense of loss, and the towering rage. It hadn't surprised him, then, how furious he was, because he was already realizing how much he really cared for the yellow and pink girl. When he saw her face, trapped in the television, he could feel a tiny piece of his heats splintering off. Because it was his name he was mouthing. Not "Help me!" or "Please!" like so many of the others, but his name. "Doctor." She was calling for him.
When he left, foolishly, to investigate the hole to the center of the rock, on the impossible planet. He had been lured by the unknown, but he should have know better. She nearly got sucked into a black hole because of him and his stupid, idiotic curiosity.
When he left her in an alternate reality, a completely different universe. He had shown her the stars, and then he stole them away from her.
And finally when he left her with his other self, on that dark little beach in Norway. She would be able to live out a happy life, with someone who could live it with her and always be by her side. Unlike him, who would have watched her grow old in front of him, if she hadn't died from something else first. He was leaving her to a better life than anything he could ever give her. Somehow, that knowledge did nothing to ease the pain in his chest.
And now he stands here, in the shadows of this tiny street, and he can feel his hearts breaking again. Although somehow this feels different from all the other times, and all the other fractures in his beating hearts. This break splits his hearts in two. He looks at her, the long blonde hair, the glittering smile, so kind, even to a stranger sulking in an alleyway. Soon he'll be gone, and some other stranger will take his place. Nothing will be left of him but memories. But her, she has all of that still in front of her. And although he cannot count how many times she's been hurt because of him, he hope, oh how he hopes, that it was worth it for her in the end. That she's now happy.
And that's the thought that keeps him going, as the pain wracks through his body and he doubles over. That maybe- just maybe- it was worth it. And, he thinks as he stares at the girl in front of him for the last time, he knows. She was worth it for him. A thousand times over.
"I bet you're going to have a really great year."