Gerascophobia; fear of growing old/aging.
The sound of running water woke Rose Tyler from a not so deep sleep. The bed next to her was empty, sheets shoved halfway down the bed, and she wondered if maybe they had been moving too fast for the half-human Doctor to be comfortable with. She certainly didn't have a problem with it, but maybe….
Rose swung her legs out of bed and pulled on her dressing gown before going to find the source of the running water. The tap in the bathroom adjoined to her - their - bedroom was running on full blast, and she turned the water off before hearing heavy panicked breathing coming from the bathroom floor behind her. She turned, and oh there he was, sitting with his back up against the wall, hands clenched in his positively great hair, knees drawn up to his chest. He didn't seem to notice the loud slapping of water hitting ceramic had gone; he just sat there, gasping and rocking slowly, body hitching with every suppressed sob.
"Doctor…" Rose whispered, crouching next to him and placing a careful hand on his knee. The half-human Doctor flinched away automatically from the touch, but allowed it as soon he realised it was Rose. Her heart just broke seeing him like this, so she knelt on her knees and gently pulled him into her embrace, guiding his head to rest on her shoulder as he clung to her.
These were all new things for the Metacrisis; nightmares, night terrors, human hormones running rampant around his new unfamiliar body. He was still convinced he could go without sleep for a good couple of weeks like the real Doctor, but he never made it. Exhausted, he would fall asleep almost anywhere, then wake shaking with a cold sweat and nightmares of headstones, coffins and a broken down rotting TARDIS crumbling away in the corner of a graveyard. It was only more incentive to try and go without sleep; he had experienced nightmares before, in his old body, but never like this. Never did it have such a profound effect on him, and it was terrifying. All those nights he'd comforted Rose after a nightmare; if this is what she'd gone through, then once again he had severely underestimated her strength. Because here he was, curled up on a bathroom floor, crying because he was scared. Of nothing but getting old. Of aging. Of dying, as a human, not regenerating as he had always done. The severity and finiteness of a human life had suddenly hit him, and his insecurities about the topic had wormed their way into his deep exhausted dreams.
"Shh, it's okay, you're okay…" Rose crooned gently, rubbing the Doctor's back with one hand, massaging his head with the other.
"How d'you deal with this?" the Doctor managed, his voice hitching through the tears. He sat back, and Rose cupped her palms over his cheeks, brushing away tear tracks with her thumbs and giving him a sad smile. She pressed her lips to his forehead once before answering and dropping her hands.
"It's being human," she said simply, lifting her shoulders slightly in a shrug. He shook his head, a hand managing to run its way through his unruly hair once again.
"It's awful," he muttered, and she slid her hand into his, linking their fingers and lifting them so the back of his hand pressed against her cheek. He looked up at Rose, his wonderful Rose, gazing at him with such love and understanding that he longed to be everything she needed with his whole being. Not anyone - Time Lord or Metacrisis - who cried from nightmares about something so natural as getting older. He gripped her hand back and shook his head at himself, a self-depreciating dry chuckle coming out before he could stop it. She didn't laugh, only gazed at him with those big caring eyes, perfectly patient. The Doctor sighed, and nodded. "I'm okay."
Her hand found his cheek once again, and this time he pressed into it, holding it there. "You wanna talk about it?" Rose asked gently, and he glanced down.
"Same old stuff. Death. Dying in general. Humans are so fragile, Rose, why? So many little things can happen, one swipe and you're gone forever! These lives are so short, and it just-" he cut himself off again, angry bitterness at his impossible situation swirling in the pit of his stomach. Rose's eyes grew misty as she listened; how she wished she could be more help than just sitting there! But all she could do was nod, and keep swallowing to keep the empathetic tears at bay. He saw them anyway, and the ghost of a sad smile played across his features.
"No Rose, don't cry," he pleaded, and this time it was him that pulled her close.
"It's not fair on you," she mumbled, sniffing into his shoulder. "It's not fair, an' I want to help somehow-"
"Rose," he interrupted, and she fell silent. He lifted her chin slightly, meeting her watery eyes. "You do help me, more than you know," he told her earnestly, lovingly. He couldn't think of a better way to describe it off the top of his head; he could only nod and say it again. "You're the only one who can make it better, and you do." The corners of her mouth lifted slightly, and she leaned in to kiss him, cradling his head in her hands as their lips met. He in turn ran his fingers through her blonde hair, tangled from sleep but in his mind, still utterly breathtaking.
"I love you," she whispered, wrapping her arms around his neck, wanting with all her heart to be able to take away his nightmares. The Doctor kissed her forehead once more before hugging her back; he pressed his face into her neck, breathing in deeply and taking her in. The faint smell of her strawberry shampoo lingered, and it calmed him. He took another deep breath, then leaned back to brush a strand of blonde away from her face.
"Bed time, then, yeah?" he questioned, almost nervously, and Rose nodded, trying and failing to stifle a yawn with the back of her hand. They rose together, and for a change it was the Metacrisis Doctor who led Rose to the bed, lay down first and tucked them in once she had slid under the sheets. Rose yawned again, but propped herself up on her elbow and stroked his hair gently, running her fingers through the wild brown mane. He eyed her curiously, and she nodded.
"Go to sleep," she murmured softly with a smile, thumb massaging his scalp. Her heart lifted as his tired eyes drifted closed, and his fist that was clenched around the blankets loosened. She waited a little while longer, never ceasing in lovingly caressing his head. When she was sure he was sound asleep, only then did she take her hand back and lie down. "I love you, Doctor. Always," Rose whispered, pressing her lips to his warm forehead once more before snuggling down under the duvet, cuddling up close to her one-hearted Doctor and falling asleep.