My Little Pony: Friendship is Magic - Celestia's selective memory

your lucky to remember things that even happened 10 years ago…

She kept a Crystal to remind her that it happened.
Just like she keeps Discord in her Garden to remind her, He exsited, and how she made an imprint of Luna/Nightmare Moon on the Moon, just so she could remember that, She has a Sister.
Celestia probably has to be reminded of such things that have happened long ago other wise she forgets.
Her Selective Memory of The Changlings and Parasprites show this.