Difficulties and Challenges

What is one of the most difficult and challenging parts of your teaching?  If you could assure that your students left your classroom with ONE thing what would that be?  What is it that makes this such a challenge?  I am not looking for answers here as much as thoughts about one thing you would like to be different.  It could be anything...such as "I wish my students would talk more in class discussions" or "I would like to have better classroom control."  What is the one thing that keeps you up at night and makes you crazy?