Normal to have a filling behind a front crown, or to have a mouthpiece for life, because of the crown?

If you have what appears to be a filling on the back side of your crown it could be that your tooth had a root canal through the crown, which was then covered over with the filling rather than making you a new crown.  Or, it could be an area of the porcelain that has been chipped or reshaped by your dentist to make your bite even.  Usually nobody sees these areas where the metal shows, so of little consequence.  A third reason is that you had some recession or cavity around the margin of the crown and the dentist filled that area, again, rather than making you a new crown.If your dentist has recommended and made you a night guard, then he suspects or knows that you clench or grind your teeth, which can cause substantial damage over time.  Protecting your teeth with a night guard will help prevent wear of teeth, breakage of teeth or dental crowns and fillings, and even help prevent having jaw issues over time.  Whether or not you should continue the night guard is a discussion your dentist should have with you and not something any dentist can determine without seeing you.