As Twilight predicted, Sunset Shimmer gave out a lot of apologies after the Fall Formal.
As Princess Celestia told Sunset long before --when she dismissed Sunset as her student-- apologies aren't always enough. Sometimes actions have consequences that can't be escaped with a simple: "I'm sorry."
Sunset remembers Celestia's words after a run in with fellow students --ones who weren't satisfied with mere apologies-- leaves her lying on the ground, in pain, in a Canterlot alley.
This wasn't originally intended to be a quiet thinking and reminiscing piece, it just sort of turned out like that. Ditto on it being both long enough and complete enough to be a legitimate one-shot.
As noted in various places, my depression has been . . . pretty much killing my ability to do anything remotely creative. So when something breaks through that it's very, very welcome. This was such a thing.
It's simple and short and exactly as straightforward as the above description implies.