The idiom cry over spilt milk means to be upset over something that has already happened and cannot be changed or when you complain about a loss from the past. It also means to be unhappy about what cannot be undone or to dwell pointlessly on past misfortunes. Usually this phrase is said as "it's no use crying over spilt milk," which means that getting upset over certain things, like spilled milk, is not going to fix it.
Example 1: I know you wish that you'd handled the project more efficiently, but there's no use crying over spilt milk.
Example 2: He is always crying over spilled milk. He cannot accept reality.
Example 3: It can't be helped. Don't cry over spilled milk.
Example 4: My kids were upset because they had burnt their toast, but I told them that it's no use crying over spilt milk and to just make some more.