Celestia uses "The Magic Inside" to cope with the banishment of her sister.

"The Magic Inside" is written by Daniel Ingram
Original Animation is "Lullaby for a Princess" : https://www.youtube.com/watch?v=i7PQ9IO-7fU
by Warpout: https://www.youtube.com/user/WarpOut

I do not own "'Lullaby for a Princess' animation" nor the song used. The clips and characters shown and mentioned are from My Little Pony which is owned by Hasbro of which I am not associated with.