Some tips, since everyone has covered the basics:
1. Don't apologize. You bring your own food and enjoy it, if other people are offended by your health problem it's their issue, not yours.
2. Don't be forced into eating something you aren't sure about. People generally won't do things like, "Oh but Mr. Diabetic, it's just a little sugar in this cupcake, you'll be fine!" "Oh but Ms. Peanut-Allergy, it only touched a peanut a little, I can just wipe it off!" Not OK there, not OK for Celiacs.
3. Find joy in the little things.
4. You will mess up, it sucks but it will get better again.