He knew, the day it happened. He'd felt it in his two hearts. He'd regenerated three times since he'd said goodbye to Rose Tyler and his other self on Bad Wolf Bay. It had been many, many years since he'd seen her smile, since her scent had filled the TARDIS. Many years of wondering and imagining what she was doing.
He was traveling alone now, walking down the streets of London in the early 28th century when it happened. It was Christmas time and it was snowing. Everyone was gone now, the Last of the Time Lords was now alone once again and on this day he knew the girl he once loved, still loved was no longer anywhere in the universe. She was gone, her shinning eyes, her impish smirk and golden hair, all gone.
He often imagined what her life had been like with his other self. The wedding, she'd have wanted a small one he thinks, and he knew himself well enough to know he'd want to give her exactly what she wanted. It would have been just the family, maybe a few close friends. He imagined Rose's first pregnancy, her stomach round and face glowing, his other self being over protective, talking to her belly as they snuggled on the couch. He imagined their TARDIS as it had finally finished growing. His other self would take her to Barcelona, not the planet but the place. They would teach their children to fly the TARDIS when they were old enough. He imagined their life full of love and it brought him joy.
Now, as he walked down the snowy streets of London he imagined her in his other self's arms, curled into his chest, aged and beautiful, asleep as death took her. He imagined his other self running his fingers through her hair as he too lay next to her, old and near death himself. He wouldn't want to live without her; they'd spent so many happy years together, why would they spend anytime apart? He imagined his other self whispering soft loving words to her and pressing a kiss to her forehead as he finally let death take him only minutes after it took her.
The Doctor eventually stopped to sit on a snow covered bench to let the reality of it hit him, as he let a few tears fall for his lost love. He was happy for her, oh yes. He'd given her a life with him that he himself hadn't been able to offer. He was glad she'd loved and grown old with his other self but that didn't mean he didn't grieve for his loss of her. After a few minutes The Doctor stood and began making his way back to his old ship.
Little did the Doctor know that in an alternate universe, one that had held the pink and yellow girl he'd been so fond of, all those things he'd imagined had indeed taken place, Rose Tyler and her Doctor lay in each other's arms as if they were asleep.