"And how was that sentence going to end?"
He looked deeply into her eyes, taking her in. Her beautiful face and her cute little nose and her dazzling smile. He wanted so badly to press his lips to hers; He wanted so badly to hold her in his arms, and whisper his promises of love into her ear. But he knew he never would, and never could. If he told her how he really felt, how could he ever leave? How could he ever tell her, and then just up and leave? He felt the words forming in his mouth. Those three simple words. He couldn't stop them. They bubbled up inside him, and he tried his hardest not to say them as tears pooled in his eyes, just barely succeeding.
"Does it even need saying?" he said, longing for just one kiss goodbye. But he couldn't do it. He couldn't hurt her any more than he already had.
"And you?" she turned to The Other Doctor. As he leaned towards Rose's ear to whisper into it, The Doctor already knew what his clone was going to say. He watched as Rose's face changed slightly, as she leaned back, looked The Other Doctor in the eyes, and pulled him close as she planted a kiss on his lips. How odd it was for The Doctor, watching the very scene happen that he'd already imagined so many times. He turned quickly and rushed back to The TARDIS, where his redheaded companion waited for him with a sad smile. He kept his head down towards the millions of tiny grains of sand, as the tears rolled down his cheeks. He stepped into The TARDIS and pushed right past the open arms of Donna, slamming The TARDIS door shut behind him. He walked to the console, flipped a few levers and pressed a few buttons. He then sank to the ground, letting out sobs of pain and dispair as he pulled his knees to his chest and buried his face in his hands, wishing the big blue box would leave him behind. Wishing he could just stay with Rose.
After a few seconds, he felt a pair of arms wrap around him gently and Donna's voice whispering "It's okay, Doctor. It will all be okay." This made The Doctor realize how childlike he was acting, and it only made him sob harder. He wanted to stop. He wanted to be strong. But more than anything, he wanted to be with Rose.
He pictured Rose and his clone years from now, with a family, a proper family. He pictured them growing old together, smiling at each other, never leaving the other's side. He pictured her smiling face, looking up at her New Doctor like she'd beamed at him so many times before. Rose got her Doctor.
He just never got his Rose.