"You always worry about everything."
Translation:Tú siempre te preocupas por todo.
I have a book called Correct Your Spanish Blunders by Jean Yates. They spend a lot of time talking about prepositions. You might want to consider buying it. I bought my copy on eBay for just a few dollars.
Prepositions don't have direct translations, you have to memorise which one is required by each verb. The complement of preocupar can only be introduced by por or de.