One letter of many

Dear Princess Celestia...
"Please don't apologize for telling me these things, nor should you feel guilty. I've lived a full life and I have no regrets. And no need to sound so fatalistic! When Applejack's grandmother passed, she was one hundred and sixty! I'm only pushing one and twenty. My bones may be brittler, my coat and mane a touch faded, but I still have more than a few years in this wrinkled hide.
But just the same, I know that you wouldn't write in such a way if it wasn't important to you. Even though ordinary ponies may come and go in your life, I want you to be strong and happy, to be the beacon of love and hope that all Equestrians look to.
We keepers of the Elements of Harmony even today have to strive to live up to our virtues. But you, Celestia, you *exemplify* them. You don't need to tell anyone how to live. You simply show yourself as an example and let them choose. I know how you think, my teacher, and it's always plans within plans, strategy and gambits. You nudge everypony towards salvation and well-being and ask for nothing in return.
Though I feel some years yet remain for my friends and myself, there are some things I want to tell you too.
My parents, rest their souls, knew that sending me to your school was the best opportunity for me. But they also knew their contact with me would become limited. In the time I spent under your loving tutelage, I came to love you back as a foal loves their mother. You were always there, encouraging me when I failed, drying my tears when I felt alone and pushing me to excellence, even when I was at my poorest.
But it wasn't just that, was it? You loved me. Like a mother loves their foal. Even I, with my vast knowledge of wordcraft, cannot articulate the strength of the bond.
Take heart, my teacher. Please...
I was meant to be born, meant to become the Element of Magic, meant to help Equestria, meant to grow old and sooner or later, meant to die. That is simply the way things must be and I am utterly at peace with that. After I proved the existence of the soul, I have no fears. I know that I will move along the road. Perhaps, yes, to a place you can't reach, but please, my dear, sweet, wonderful mentor, don't despair.
There will be others. More gifted unicorns, Earth ponies and pegasii.
There will be many more who will look to you for guidance as you take them under your wings, who will need your love and your wisdom. They won't need you weeping over a unicorn who's been dead for dozens... then hundreds... then thousands of years.
You are the greatest force for good in Equestria, you and your sister both.
You may carry a burden, but you should know that we ponies who look up to you love you more than mere words written in a book or spoken in even the most earnest form can express.
And that is what I want you to remember, beloved teacher.
Not that I died.
But that I *LIVED*.
But that's all talk for another time! I know I have more than a few years left in me and that magic symposium isn't going to run itself. Though perhaps I could make a spell for that...?
I'll be here for you however long I can, dear princess.
For all that you gave me, let me give something back. And that is as many years as these brittle bones can muster.
For now though, take care!
With love, affection, courage and harmony,
Your faithful acolyte, learner and daughter-in-spirit,
Twilight Sparkle."