He feels it about an hour after Donna and the Doctor left, like a sinkhole in his mind- the loss of a telepathic connection.
"Um—" He faintly hears Rose's voice. "Are you okay?" He looks up—when did he sit on the ground? He had been standing up just a second ago...
Rose is standing right next to him. He blinks rapidly, confused—her voice had sounded as though she was several metres away.
"I'm alright," he says, because he is, and he knows that's what she wants to hear. She doesn't want to deal with a man with the same face as the man she's loved for years and who is almost the same person but not quite—well, he thinks so, anyway.
He's not sure who he is yet; he's only been alive for a couple hours. Well, he was alive before then, sort of—Donna just sparked him into being... Instant Doctor, he thinks, grinning. Just add Donna.
Donna. He closes his eyes tightly. His other half is missing and how am I supposed to know what to do now?,and he knows that she would've died if she'd been allowed to remember but oh, how he wants to be selfish and cling to the small fragments of guidance he had from her—the Doctor and Rose are trying, he knows, but they both shut him out because he reminds them of what they can't have—and he's sorry because there's nothing he can do to fix it, and now Donna can't remember how extraordinary she truly is and she's gone back to being the woman who misses everything—
"Mum!" he hears Rose calling. Her voice seems distorted, as though he's underwater. "Mum, he's gone white and he's started sweating. I don't know-" She's crying, he thinks dimly. Perhaps that's it- he's submerged in her tears. It would be strange to drown in someone else's tears... He's certainly got enough of his own for that, he thinks wryly. Are they his own? They're the Doctor's, certainly... But that's not really the same thing—is it?
"Talk to me, you massive idiot." Jackie. When did she get here?, he wonders. "What's going on? You're scaring the hell out of us."
He tries to move his lips to explain the hole in his head to her but it doesn't seem to work. I'm speechless—Donna would be astonished, he thinks. There's no crackle of amusement from the space where Donna used to be in his head, and he feels a sharp gasp escape him as his eyes fly open and he starts to tremble.
"Fuck," he hears Rose whisper. He feels her arms wrap around him and her hands petting his back gently and he wants to tell her Dame Rose, you're being frightfully improper, what would Queen Victoria think? just to get a laugh out of her but his mouth still isn't working how do I get it to work I don't understand so he turns his face into her neck and focuses on making his breaths slow and even until the world goes fuzzy blurred like forgotten memories around the edges and he slowly falls unconscious with Rose's hands still spread over his back.