It took her quite a while to deal with Luna’s betrayal and banishment.

“501 years! And every year I can’t help but ask the elements to take it back… but take what back? I have my sister here, my ponies are safe and happy, maybe I meant to ask them for something more? Perhaps some more games or cheese sandwiches for Luna? Lulu loves to play- sobbing”

The first months after the banishment must have been something like this.
 
But only 501 years?

Glad someone already added the ‘sad’ tag.

>501 years old
 
Me: Denial isn’t a good habit to get into, your highness.
 
Celestia: Hush, you. I am so 501. Just ask Luna over there. I’m not an old hag, am I Lulu? *eye twitch*
 
“Luna”: ……
 
Celestia: See?