I suspect "Don't cry over spilt milk" might be only part of the original aphorism. Possible roundings-off include "Crying won't put it back in the bottle/churn", and "The cat won't lick it up for you --- he doesn't like salt".

Much depends on where the milk was spilt from. In the immortal words of Willy Rushton: "Out of the mouths of babes and sucklings .... and straight down the back of your suit."