That Has Made All the Difference

http://www.fimfiction.net/story/50018/that-has-made-all-the-difference

I typically don't comment when people are generally saying what I'm thinking, but there's a few key points that the other comments aren't quite hitting that I wanted to say. First the points I agree with, an interesting concept (though questions about how RD would have joined the Wonderbolts without even having won the young fliers competition to this point, and how Nightmare Moon is defeated without the element of Loyalty, etc. However, those are secondary to this world), pretty well written overall, and just on the side I agree a lot with your interpretation of the Frost, mostly because it all comes down to how you interpret that sigh in the last stanza. But I digress.
The main issue I see is that we're not given enough to connect with Rainbow's decision and the consequences of it. The Fluttershy conversation makes an attempt at this, but there's nothing shown by Rainbow that anything is actually missed. There's no attempt at reconciling (which could also fail to emphasize it), no remorse shown by Rainbow, nothing that makes us (the readers) feel that it means anything to her. We've nothing to connect to the loss of friendship in that whole conversation, and granted Fluttershy would not likely have a lot to say, especially if Rainbow and 'Shy aren't as close, but the conversation feels stilted and wooden and impersonal without that connection.
The Pre-Sleep paragraph makes a good attempt at trying to get the readers to connect, however its lost in a very clear show vs. tell issue. We never get the emotional investment to justify the "Before falling asleep, Rainbow Dash began to cry." Rainbow hasn't displayed enough emotion to let this conjure a visceral image for us. We're left with a feeling of "Oh well" as we're told that she's crying. A deeper description of how Rainbow breaks down with some sort of build up would help this section immensely.
Again, the build up and showing of the breakdown isn't the only way to handle this. If the bluntness of the "Rainbow began to cry" is something you want to go with, then there has to be a bigger build of what Rainbow thinks she's missing and what it might mean to her so that when the readers arrive at the crying they can connect with what Rainbow is thinking and feeling. To be honest, giving the readers more of that connection would be better, regardless.
Finally, the finality of the last statement is great, but we're not given the consequences she's facing as she sees it. What we have of her consequences is that 1.) she can't make the party Fluttershy invited her to, 2.) and I quote "the life she had chosen for herself". Saying that the consequences of her decision is her life is practically an empty statement and certainly doesn't evoke anything in myself personally. It's simply a true statement. Without any consequences to connect to Rainbow's life, saying that "You can never escape the consequences of your actions" doesn't ring true enough. Heck we've all had to tell a friend at some point, "ooh sorry I can't make that I've got work, or a paper, or a project, or a family trip, or something". So seeing Rainbow turning down a party invite because of her job, a performance no less, is completely reasonable. We, as an audience, know how her life would be had she gone the way of the show. Rainbow does not know this, so the only consequences she can connect with are the ones she imagines for herself. And unfortunately none are described here, so we are left with the impression that Rainbow honestly doesn't care. Which is a legitimate response, but does not evoke sadness nor connect with living with the consequences if you don't think about them and don't care.
Anyways, I just wanted to get a little more specific because the concept is intriguing and there's a lot of potential here. I certainly am not here to dictate how any writer should write. Even if you disagree, which is certainly your right to, I hope that you'll take some of my broader points under consideration, regardless. I think the general critiques warrant some consideration even if you disagree with the specific examples listed.
Best of luck to you.
-Duskrider