The Doctor stood before the central console of the TARDIS. She was gone. Rose was gone forever. She was with Mickey, her mum, and her new dad. And a little baby sibling. She had her own life, her own adventure. She was going to live on without him. Rose could have come through the gap, the Doctor knew it. She could have come with him, come adventuring again across their galaxy and through their time. But Rose had to stay there. She had to live her own life, safe from his meddling ways. She had to be safe. Maybe in a few years, she would have her own baby. She and Mickey were well-matched, after all. She'd settle down, have a family of her own. She'd die old in her bed, surrounded by children and grandchildren.
Rose dead. That sent a shock through the Doctor and he put a hand to one of his hearts. He'd thought Rose was dead before, but it was different from this. This time, he couldn't save her ever. There were no Daleks to beat, no Cybermen to overcome. There was no ransom to be paid this time. He couldn't save her when he himself had condemned her, and it hurt like the Doctor had never hurt before.
Suddenly, the Doctor realized something was off. His hand was on his chest, but he couldn't feel the heart on that side. The other one pumped fine, but his right heart had stopped.
"No way…" he whispered. He checked again. Still nothing. The Doctor sank to the floor, hollow. It was gone. His other heart, his giving-heart, was gone.
There had been rumors among the Time Lords that such a thing could happen. The Doctor had never seen it himself, but apparently it was true. When a Time Lord met his mate, his one true mate that could be anywhere in space or time, his giving-heart was theirs. When the mate was gone, the heart yearned for them. When the mate was gone forever, it stopped.
Rose was his mate. Rose had been and always would be his forever-mate. She had his heart, and she didn't even know it. His Rose, his sweet fierce Rose, had been his. The universe had designed them to fit together. She was the peanut butter to his jelly, the edible ball bearings to his cupcake. And she was gone forever. She was off living her life, holding his heart, and he hadn't even told her. He hadn't told her he loved her.
Oh, how he loved her. He hadn't known what to do with himself before her. He had gone around saving worlds, that was just his way, but it had been lonely. Lonelier than the Isolus, because he was the last of his kind. No one had held his hand when not everybody lived through his meddling. He'd been alone. But with Rose, everything was better. Good had been better, suns were brighter, colors vivider. She'd been with him to celebrate the better and hold him together through the worse. Looking back, there had never been one like her, and there never would be again. He would be alone forever.
And so would she, he realized. If she had his physical heart, he had her metaphorical heart. She would never love another. He would haunt her forever and she would never be rid of him. He had ruined her. The Doctor slumped against the center console of the TARDIS and cried.