What is the meaning of the idiom A mill cannot grind with water that is past?

This isn't an idiom. It is a proverb or saying because it means just what it seems to mean. Think about it. How could you use water if it's already gone down the river? If someone is always dwelling on the past, they're getting about as much done as a miller trying to use water that's already flowed past him. In other Words, both of them are wasting time and energy on something that's just not going to happen.