Keep up the practicing in a mirror. If you do it diligently enough you will definitely be able to pull off those same facial expressions without the mirror and know you are doing it right. That's where the major practice comes in. What I often tell other models to help them with facial expressions is to make a list of different emotions and write them down on a piece of paper. Then get a friend to read the expressions out loud while you stand in front of a mirror. When they name the expression, then do it in the mirror. Then have them go through the entire list. To mix it up, have them go out of order so you don't memorize the order of the facial expressions.
The best way to exaggerate your facial expressions is to do just that--exaggerate! Often if you feel like you're being silly, chances are you're doing it right. You want to go to the extreme if you are having trouble with emotions and expressions and have a habit of holding back.
Also picture scenarios that go with each emotion and place yourself in that moment. For example, if you have to do a surprised expression with your mouth open, picture that you won the lottery or that your favorite male celebrity has just knocked on your door and you open the door, see him, and BAM there's your surprised expression. Coming up with little "stories" to match your emotions can help you get into the moment a bit better.
Again, it all comes down to practice and time. Some people are able to get it right away while it may take some time for others. Good luck and congrats on booking work in the commercial and promotional field!