Dear Princess Celestia,
I am so sorry for this whole mess with Rarity. I know the symptoms of cutie mark failure syndrome quite well by now, having seen it in person so many times. I should have seen it coming, and that it was a very severe case. Though I don't quite understand why it was so severe, but I'm sure that's just due to my own inattention.
I won't fail you again. I'm going to make sure I always have time for my friends, no matter how important my research may be, even if it directly affects them. I think the thing I learned this week about friendship was more a reminder than a new lesson. That friendship is so important, so vital, that sometimes it can even save the world. Hopefully it's one I won't forget again.
Your faithful student, Twilight Sparkle.