AN: Inspired to write this after watching "The Water on Mars". Of course, I do not own anything.
He had made the wrong choice. For a genius, he certainly could be a right idiot. The Doctor realized now – far too late – that he had made a grievous error.
He thought he was doing the right thing; he left Rose with a version of himself, one that needed her love and calming influence. He thought he had received enough of it himself that he could allow her to mold his clone. But it wasn't true.
Without Rose, he lost his conscience, his compass, his sense of what it was to be a Time Lord. His hearts still beat, but they are hollow and empty and despite the love he felt for his other companions, none of them could fill the hole left by Rose.
It had been so painful to leave without her, but watching her age and die would have been painful too. He should have known he wouldn't have had to watch that happen with his eyes; they would be some other doctor's eyes. His clone, his damn clone, he was worried about what the metacrisis would be like without Rose's influence, but he didn't fully understand, not the way he did now at the end.
His clone carried all his memories of the love he shared with Rose. Though his clone was born in fury, he still could remember how she had looked at him and how she could make his hearts stutter. But more importantly, his clone was powerless. He was half-human, which meant there was very little damage he could do.
That was the mistake; forgetting that he was the one still balancing the power to destroy worlds. Without his Rose, the power had gotten out of control. He had decided he made the rules and now he was being punished. He could have stayed around so much longer, made fewer mistakes, if a certain yellow-haired girl had been by his side.
Now at the end, the weight of his mistake weighed on him. He had tried to be noble, but it was all a waste, for him and the world. It would have been better, safer, if she had stayed with him. If he could have told her how much he loved her. Now he never would.
Before he left his body and started out again, he resolved to see her one last time. Maybe he couldn't fix the fact that he left her, but he could give a little nudge to make sure she went with his big-eared self. And, he could finally admit to himself, he just wanted to see her. Even if he couldn't hold her or touch her or even tell her how much he loved her, he wanted the last person that he saw in this regeneration to be her. She was the first to see him and now she would be the last.
"I don't want to go." The words were true, but he knew he had to go. This regeneration was broken beyond repair, driven mad by the emotion of love and the loss of that love. As the golden light consumed his body, he apologized to himself, to Rose and to the rest of the world for all the mistakes he had made. Mistakes that could have been prevented had he only given into his hearts. Next time, he would give into them, whenever, however, whatever they needed. He would not hold back his love and affection. He would learn from his mistake.