There’s nothing wrong with being quiet and if you stay with your boyfriend you’ll have to deal with them eventually. It may sound weird but I’ve found positive affirmation to be very effective. If you decide to go try writing something like “I will relax and have a good time at Christmas dinner” about 15 times a day until you go. You have to write it by hand though. Typing doesn’t seem to be nearly as effective. I’ve used this technique many times with good results.