It’s no use crying over spilled milk

IT’S NO USE CRYING OVER SPILLED MILK – Prov. Do not be upset about making a mistake, since you cannot change that now.

Example:
I know you don’t like your new haircut, but you can’t change it now. It’s no use crying over spilled milk. 

OK, so you broke the drill I lent you. Don’t cry over spilled milk.