(n.) A mistake that cannot be rectified.

when you've done something so cringe you can't stop replaying it in your head and it stops you from getting on with your every day life

Something that you shouldn't make too big of a deal over

Something mildly bad/unpleasant which has already happened, and nothing much can be done about. Usually referred to in the context of "stop crying over spilt milk".

Something that you should not cry over.

1. A problem or situation that has become out of control or irreversible.

2. Milk that is no longer in its container.

Something unimportant, especially the former significant other of the person you're now dating. Refers particularly to the casualties resulting from boyfriend/girlfriend stealing.