Dear Sunset Shimmer,
I miss you, and I am sorry I sent you away. I hope you are well, wherever you are right now, whatever you're doing, and I hope that you know you are always welcome home.
I understand it's been a long time since my last letter, but recent events have made me realize that... well, everypony has their oddities and foibles. A temper and sense of entitlement really aren't that bad, all things considered. I hope your time in another world has helped temper the latter, and settle the former, if you'll forgive the wordplay.
I imagine you've met Twilight Sparkle by now, given that I don't really put anything past her ability to achieve (for better or worse). I've taken her under my wing as a new student, but again, you're always welcome back, Sunset, she hasn't taken your place. I'm writing because... I've been reflecting on the good pony at heart I know Twilight can be, and I remember the light and warmth in that little filly I had taken in those years ago, and realize that I have been unfair to you. If you do return, there are some things we need to work on, but you're worth the effort, just as Twilight is.
I miss you, Sunset.
Princess Celestia