What's the Difference between Nervousness and Excitement?

I'm about to embark on a new adventure in my life.  Tomorrow I begin teaching my first college class.  I'm really excited to be taking this step, but I'm also nervous as hell.  I'm realizing tonight that I'm having a hard time distinguishing the line between my nerves and my excitement.  Have any of you been in a similar situation?  I mean I'm excited to have this opportunity, especially since it is something I can cross off my life list, but I'm nervous that I'm not cut out for it.  But maybe part of the excitement comes from facing my fear and being able to overcome it.  And I should state that I'm not really afraid of the teaching part, but this is a college class and there's a lot more at stake than there ever was when I taught middle school.  On the other hand, maybe my nerves come from the fact that I'm so excited about this new opportunity that I just want to make sure I make the most of it.  I don't know.  I can't tell.  What I do know is that tomorrow is the beginning of a 16 week adventure in which I'll be fulfilling one of my dreams.  And whether I'm feeling nerves or excitement, I have to fight through it and teach the class to the best of my ability and try to provide my students with a memorable experience that they will take with them long after the class has ended.  So wish me luck and stay tuned for my reflection post after the class ends.