don't cry over spilt milk
Definition from Wiktionary, the free dictionary
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- It is no use worrying about unfortunate events which have already happened and which cannot be changed.
it is no use worrying about unfortunate events which have already happened
See also 
- Gregory Y. Titelman, Random House Dictionary of Popular Proverbs and Sayings, 1996, ISBN 0-679-44554-4, p. 63.