Story from source.  
Artist Description  
“Dear Princess Celestia,  
Throughout the many years that I have studied friendship in Ponyville as per your request I have experienced so many joyous and sorrowful moments with my friends. Though I feel a growing relief now that I feel my life beginning to pass away; I also have a growing concern about leaving you, Princess Luna and Spike behind to overcome this without me there to help you. The pain only worsens with the thought of Spike who is losing the closest thing he has to a family and is alone in Ponyville tending to the library. Im beginning to diverge from what I want to tell you. I want to thank you Princess Celestia for allowing me to spend all those years in Ponyville with my friends and may all the warm memories we forged together see you through this difficult new stage in both our lives.
 
Forever your faithful student,  
Twilight Sparkle”