Disclaimer: The dialogue isn't mine so yeah?
"Can't you come through properly?" (That's her desperation, but of course she knows the answer.)
"The whole thing would fracture – two universes would collapse." (If they were traveling and this was happening to two other people, and she just wanted to help, he would hesitate, or lie – "Yes, of course they'll find a way to be with each other again, Rose. And they'll do it without destroying the universes, and everything will be fine." – and they'd walk back to the TARDIS, shut the door and move on without thinking of it ever again. But this was happening to them, now, and he loved her so much it was killing him, so maybe he'd tell her, finally. If they had the time.)
"So?"
And then he laughs because he's thinking the same thing. Of course he's thinking the same thing – they think the same way, ever since he changed for her. (Or maybe she changed for him, who can tell?)
She wants to laugh, wishes she were joking, wishes she gave a damn that if the universes collapsed so would she, so would he, so would her whole family. She just wants him. Just wants him with her.
Of course, there's no way he'll forget her, not ever. She taught him things, life things, like the fact that you don't need guns to solve problems, like the fact that things can get better, like the fact that even a broken man can love again. (Again, because she wasn't the first, of course, but he silently vows that she'll be the last.)
And then she taught him other things. Things so very, very human that it terrified him for the briefest of moments. And then it was all so wonderful: shirts flung over balconies and leaning against the console or back onto that one chair, going through the TARDIS rooms one by one by one by one by one…
Anyway, he reminds himself, they only have thirty seconds left.