The song is from season 4 episode 8 of MLP:FiM.
Main song: http://youtu.be/k6uWdyshWmM | ♫ Lyrics for the song:
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[Rarity]

Oh, Manehattan, what have I done?
The thought of Fashion Week was fun
But I went way too far
My friends gave to me in ways so kind
And I gave them nothing but a hard time

And now alone I stand
And now alone I stand

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"My Little Pony" is owned by Hasbro,
"MLP: Friendship is Magic" is created by Lauren Faust.