Facing the loss of his TARDIS, the Doctor didn't know how he felt about being spared in return. There was Rose, of course. She was unmistakably his idea of a reward, she'd been all he wanted to see before he regenerated and would no longer reflect on her with the same fondness. He would be a different man and Rose a parallel universe away. The Doctor had gone through enough companions to know that, though they were never forgotten, their place diminished in his hearts for the distance.
Granted he hadn't consciously realized that until he faced Sarah Jane again with this face.
Clasping his hands before his face, his eyes glassy again, the Doctor attempted to steel himself. He didn't have any answers for her and that ate him up inside. Oh, he would get back on his bouncy game in no time, he knew. No one hid their pain like the man born of regrets and the utmost love for the woman beside him, but it would take a bit more time than usual.
"I don't know," he answered honestly, a lump in his throat unpleasantly swallowed.
How could he keep his promise to Jackie now? Looking at Rose, closed hands partly obscuring his distress, he felt the heaviest of questions coming before he heard them. They were from vastly different points in time, so that meant old leather and ears could turn up, a man with fondness for unnecessarily long scarves could arrive wanting jelly babies, or even a man he had yet to meet (by his recollections). Slowly, the Doctor's hands fell between his legs--not before rubbing his so very tired eyes--and he thought it best to get the worst news out of the way now.
He'd rather she know now than find out by a brand new face calling himself the Doctor showing up.
"It was and it wasn't," he answered ambiguously, still contemplating a proper way to say it. "I'd say it snatched me from my own timestream. I was gone, Rose. Long gone. Got everything I deserved, you could say, in the end. Some other man is the Doctor now, then--somewhere. Has to be, we're from far apart, you and I. That means the timelines aren't broken completely, they've continued, quite a cause for concern."
Arms folded across his knees, he looked off into the distance, focusing on nothing in particular. Listening, yes, but his mind was miles away then. An impossible grimace on his face, the Doctor wrestled with the darkness of his disposition. This was neither the time nor the place to be depressed or bring her down. He'd gotten something brilliant in the end, something he didn't think he had it in him to ever tell her. He was so bad at telling her anything that needed saying.
So, he forced himself out of the slump bit by precarious bit with a flattened smile. "Not a problem though, been in similar scenarios! I'll get this sorted. I will," he nodded as confidently as he could, still fearful of looking at her now that he'd implied his regeneration to her. "Loads of promises, I've got. Hah."