((Prompt: Be careful what you wish for, because you just might get it.))
Guilt. Fluttershy was no stranger to the emotion. Ponies cracked jokes about how she would apologize for apologizing too much. The words 'I'm sorry' came to her tongue easier than her own name. Her friends always reassured her that she had nothing to apologize for, that she had done nothing wrong. Sometimes, they were right. It was just her over-cautious nature seeking to internalize things that she had no control over.
But there were other times when she deserved the guilt. Sometimes, she had done something so terribly, terribly wrong that just apologizing for it was utterly insufficient. She had to apologize for everything else, because it was all she could think of to do. She was sorry. So very sorry. Fluttershy was a sorry pony.
Guilt lanced through Fluttershy's heart like a knife as she watched Rainbow Dash strain to reach the cupboard. There was something so completely wrong about seeing Rainbow reach with her hooves for something above her head instead of simply flying up to get it. Her eyes were set on the wooden handle with a single-minded focus, and she made quiet grunts of exertion as she stretched for it.
Fluttershy couldn't bear it any longer. She flew up and opened the cupboard, fishing out the glass with Rainbow's cutie mark and placing it in her hooves.
Rainbow Dash scowled at the cup. “I had it, Fluttershy,” she said
More guilt. Fluttershy dropped to the ground and folded her wings to her sides. “I'm sorry,” she said, casting her eyes to the floorboards. “I—I should have asked if you wanted me to get it for you.”
Rainbow's expression softened. She set the glass on the counter and drew Fluttershy into a hug. “Hey.” She gave her a tight squeeze. “It's okay, 'Shy. You're just looking out for me. I appreciate it.”
Fluttershy put on a smile for Rainbow's sake. “Would you like me to get you some water?”
Dash put on a smile for Fluttershy's sake. “Sure. I'm a bit tired after my run with Applejack.”
Fluttershy twisted the tap, and a stream of water came out as Rainbow trotted out to sit on the couch. It wasn't as though Rainbow couldn't fly anymore. It was just not like it used to be. Short, unsteady bursts were all Rainbow could manage these days, and they hurt her when she tried.
The glass filled. There had almost been enough time to forget now and then. Fluttershy could look over at the blue mare perched in her sitting room, and almost pretend Rainbow hadn't once dreamed of being a Wonderbolt. That she was satisfied living here with her.
There was only a hint of a shadow in her eyes left these days to tell the tale of just how close she had come, only to have it stripped away from her.
Rainbow had been accepted and everything. Pinkie had thrown a party, and everypony in Ponyville had come to congratulate her, and wish her the best before she packed up and moved off to Canterlot to fly full-time with her foalhood heroes.
Fluttershy had been happy for her, too, except... well, Canterlot was a long way away. Dash had promised to keep in touch, but Fluttershy remembered making the same promise to her parents in Cloudsdale.
So that night, she had made a wish. It had been a horrible, selfish wish, and she hadn't meant it, but she had looked up at the night sky and wished there were some way Rainbow could stay in Ponyville.
Then the train accident had happened.
The doctors at Ponyville General Hospital were truly miracle workers. Lesser surgeons wouldn't have been able to save Rainbow's wings at all. Rainbow Dash was lucky to still be able to move them, even if they told her she would never have full range of motion again.
The Wonderbolts had been sympathetic; they had offered her a position as an assistant trainer when they heard. But Dash had declined. She couldn't bear to spend her life watching everypony else take part in what had been her dream.
Fluttershy had gotten her wish. Not only did Dash stay in Ponyville, she ended up moving into Fluttershy's cottage because her cloud-home was out of reach. Fluttershy got to spend all the time she could ever dream of doting on her oldest friend, taking care of her, and wiping away her tears when she shot out of bed at night, haunted by thoughts of what could have been.
Fluttershy carried the glass of water out to the sitting room, carefully balancing it on a wing as she offered it to Rainbow. “I'm sorry it's not as cold as it could be,” she said as Rainbow took the glass between her hooves.
Rainbow Dash took a gulp and smiled. “It's just fine.”