What is the origin of the idiom "cry over spilled milk" and when to use it?

The idiom "cry over spilled milk" may be related to the idea of spilling milk, which is a common and easily remedied mistake. The phrase has been in use in English since at least the mid-19th century, and it is often applied to situations where someone is dwelling on a mistake or setback instead of moving on and focusing on what can be done in the present. In general, the phrase is used to encourage people to let go of past mistakes and focus on the present and future, rather than wasting time and energy on things that cannot be changed.