The Doctor could feel the exact moment Rose realised what his plan meant.
"I'm supposed to go."
He dropped the magnaclamps and ran to one of the computers, carefully not looking at Rose. "Yeah."
"To another world, and then it gets sealed off."
The flat disbelief in her voice stung. He wanted to grab her by the shoulders and shake her. Did she think he wanted to let her go? That he wanted to send her to a parallel universe and never see her again? His hearts were already dying at just the thought of it. Rose belonged with him.
"Yeah," he managed to say, somehow.
"Forever."
And oh, that wasn't fair. She'd used that word—the same word she promised him—on purpose. But it was his own fault for being foolish enough to believe he could have even her forever. The world was not that kind to him.
"That's not going to happen."
The Doctor couldn't look at her. She sounded so confident—that's not going to happen. But he'd felt this coming since the Olympics, two weeks ago. She'd said the same thing then, or nearly—they keep trying to split us up, but they never, ever will.
But he could see what she couldn't. He could see the way Time dictated they lose each other. He could see all the other, horrifying ways that might happen if she didn't get with her mother and Pete, back to Pete's World.
Rose, though... Rose couldn't see that. All she could see was them. He listened as she explained to her mother why she would never leave his side, and though it broke his hearts to do it, he crept up behind her and dropped the dimension hopper around her neck.
She was gone in a flash, leaving him staring at the empty spot where she'd been just seconds before. It was better this way, he reminded himself. Better that he knew she was there and safe than watch her fall into the void, or be killed by a Dalek.
It was better this way.