cry over spilt milk, don't
Also, no use crying over spilt milk. Don't regret what cannot be undone or rectified, as in The papers you wanted went out in last week's trash, so don't cry over spilt milk. This metaphor for the inability to recover milk once it has been spilled is very old indeed, already appearing as a proverb in James Howell's Paroimiografia (1659). It is sometimes shortened to spilt milk.