For me, facial expressions are mostly involuntary, meaning that if I'm happy, then I will naturally have a happy expression, or a worried one if I'm anxious or a frown if I'm angry. If I am feeling the emotion, then I naturally make the matching expression. What I do have difficulty with, is modifying my expression to suit a social situation, for example, showing enthusiasm or sadness when my internal emotion is something different. I often feel neutral/indifferent to many situations, yet I'm expected to show some kind of emotion to be socially correct - I really can't fake emotions and don't know how to make my face show that particular emotion. I'd make a really bad actress.
I am also often completely unaware of what expression I have or what my face is communicating to others, as I often have people ask me what the matter is when I'm not aware of anything being the matter, or that my face is showing such a thing.