There’s plenty more fish in the sea.
Source: This aphorism has been traced back to around 1573. First noted in the United States in _Keziah_ by J. C. Lincoln, it is found in varying forms: "The sea is full of other fish"; There are plenty more fish in the sea"; "There are more fish in the sea than ever came out of it"; "There's more than one fish," etc.
From _Random House Dictionary of Popular Proverbs & Sayings_ by Gregory Titelman