cry over spilled milk

cry over spilled milk or cry over spilt milk  {v. phr.},  {informal}

To cry or complain about something that has already happened; be unhappy about something that cannot be helped.

After the baby tore up Sue's picture book, Sue's mother told her there was no use crying over spilled milk.

You have lost the game but don't cry over spilt milk.

Compare:

MAKE ONE'S BED AND LIE IN IT,

WATER OVER THE DAM or WATER UNDER THE BRIDGE.