I wrote this a year ago, but I just figured out how to submit stories to I don't own the characters, I'm just borrowing them. Enjoy!
It hurts her to know that she can't trust him implicitly anymore.
A few months ago, she'd have believed anything he said to her. He never gave her any reason not to. She always had known that he'd put her and her safety above nearly anything else in his world, and she knew she could count on that.
But then he regenerated.
It was so hard at first, getting used to the new him. He had little quirks he hadn't had before and he spoke differently, but he was still the Doctor. That's what he told her, at least. So she listened to him and tried to understand, because she cared about him more than any one else in her life.
She got used to him after a time. Sure he wasn't exactly the same as before, but she could tell he still cared about her whenever he looked at her. Even though he acted differently and talked way more than she ever thought possible, he was still her Doctor. He'd never hide anything from her.
Looking back now, she can't believe she thought that.
Because she just met Sarah Jane.
It was a shock, to say the least. She was hurt that he had never told her about all of the others he had travelled with, and it hurt even more to think that she would become one of the faded memories he never told anyone else about.
It's a funny thing, time is. It can go by so quickly that if you blink you'll miss some of the most important things in the world; other times it can go by slowly that you think you'll die of boredom.
She'd known that already. She just never thought that she could change so much in such a short time.
He'd shown her the universe. He'd taken her to the end of her world and shown her worlds that she couldn't even believe. It had changed her, made her more aware of life. It showed her that the only way you could really live life was by not knowing how long you'd have left. She could die tomorrow, but at least she'd lived.
She's keeping track of how long she has left now.
He told her he'd never leave her, but he'd left Sarah Jane. She was sure Sarah Jane had never thought he'd do that to her. What would he have said? 'Enjoy it while you can 'cause you only have X amount of time before I'm leaving you'?
If he left Sarah Jane, surely he'd leave her as well. So she's waiting for her time to run out, waiting for the day her world ends because he takes it away from her.
She hasn't left her room in five hours. When Sarah Jane left, she just snuck out of the console room, knowing he wasn't really paying attention to her at the moment.
She's safe for now, here in her room. She doesn't think he'll leave her any time soon, but she can't be sure. She does know that she'll never step out of the TARDIS first when in lands on Earth, even if it is just for a visit.
He never told her about regeneration, he never told her about all of the others.
Now all she can do is wait to find out all the other things he's never told her.