A tidy little proverb in rhyme: if there's a cure, find it; if there isn't, let it go. More a piece of homespun philosophy than a story, and better suited to slightly older children.
From Mother Goose / Nursery Rhymes (traditional). See the whole collection.
For every evil under the sun
There is a remedy or there is none.
If there be one, seek till you find it;
If there be none, never mind it.
About this rhyme
Text confirms the remedy-or-none, seek-it-or-never-mind-it structure; described generically as an old proverb without inventing a specific origin.
A proverb turned into rhyme: every trouble either has a fix worth chasing or it doesn't, and either way fretting won't help. It reads less like a nursery rhyme and more like a piece of old, practical wisdom that happened to land in rhyme.
A useful one to say when something has gone wrong and can't be undone.
Public domain. Text from The Real Mother Goose (Blanche Fisher Wright, 1916), via Project Gutenberg. View the source edition