Sunset, Trixie and Twilight were chosen as Celestia's apprentices, but they also choose to become Knights, protectors of Equestria. They are prepared for that, but Equestria is probably not prepared for them.
Approved by the author of Simple Melodies are for Fillies comes a shipfic in the same style for your other favorite couple Twilight Sparkle and Trixie Lulamoon.
Sunset Shimmer's diplomatic visit doesn't go quite as planned. Especially when nudged off the rails by a lot of booze, some minor flirting, and an entire truckload of bad decisions.
It turns out that Sunset Shimmer's final letters to Princess Celestia before her exile were not filled with resentment or remorse. There might have been a lot more frantic running away involved. But it was definitely not Sunset's fault. At all.
Sunset Shimmer tries to explain to Princess Celestia what happened at her CSGU class reunion. And this time everything is very, definitely, not her fault. At all.
Trapped on the other side of the mirror, Twilight Sparkle has to adapt to a world without magic. Given the circumstances, that is the least of her worries.