English to German translation help, famous quote?

I'm trying to find the translation of this quote by Johann Wolfgang von Goethe



"The really unhappy person is the one who leaves undone what they can do, and starts doing what they don't understand; no wonder they come to grief"



I tried google and some dictionaries online but many of them come up with a different result. Anyone knowledgeable in German, who could tell me the right original translation?English to German translation help, famous quote?
Found it with the "was er nicht versteht" portion (what they don't understand):



Wohl ungl眉ckselig ist der Mann, der unterl盲脽t das, was er kann,

und unterf盲ngt sich, was er nicht versteht; kein Wunder, da脽 er zu Grunde geht.



It rhymes. :)