What does Sorrow not over what is lost forever mean?

Originally from Aesop's Fables (Perry 627).

It is similar to the adage, "There is no use crying over spoilt milk."

There is no point in sorrow over things that you will never be able to have/experience again.

For express references purposes, here is the full text of the fable from whence it came:

The Labourer and the Nightingale

A Labourer lay listening to a Nightingale's song throughout

the summer night. So pleased was he with it that the next

night he set a trap for it and captured it. 'Now that I have

caught thee,' he cried, 'thou shalt always sing to me.'

'We Nightingales never sing in a cage.' said the bird.

'Then I'll eat thee.' said the Labourer. 'I have always heard

say that a nightingale on toast is dainty morsel.'

'Nay, kill me not,' said the Nightingale; 'but let me free,

and I'll tell thee three things far better worth than my

poor body.' The Labourer let him loose, and he flew up to a

branch of a tree and said: 'Never believe a captive's promise;

that's one thing. Then again: Keep what you have. And third

piece of advice is: Sorrow not over what is lost forever.' Then

the song-bird flew away.