Remember that your eating plan is for YOU, not your friends and relatives. You do not owe it to anyone to overeat or consume things that are bad for you. "Polite" does NOT mean consuming things you would not otherwise eat so you can tell your host how much you like it. It also does not mean overeating so your host can feel successful. It does mean being appreciative of the things you can eat comfortably and enjoying your meal.
On the flip side, there is temptation. Some part of you really wants that pumpkin pie or that stuffing, even after you are beginning to feel as stuffed as the turkey. It can be difficult to remember how bad it will feel later when you have eaten a plateful of things you are not accustomed to in quantities you rarely consume. Do treat your inner child to a taste of whatever you love. Don't feel obligated to overdo it--don't eat a whole slice of pie or a whole serving of stuffing. If you pace yourself and taste everything that really tempts you, you can enjoy your healthy holiday and feel good about having limited your indulgence.