Scared about eating dinner with boyfriends family.. help!!?

For the 4th of july my boyfriend and his family invited me over for a family dinner and fireworks after. The thing is, I'm super strict with what I eat. I used to be not very healthy and finally i've taken control over my life and I am a lot happier with my body and the food that I eat. Tomorrow there will be potato salad (mayo, mayo, mayo) lots of cakes, cookies, sugary lemon aid, etc. I really don't want to be rude and not eat, but I know that if I do i won't be happy with myself and i'll feel guilty. I can't just sit there and not eat though! they'll think i'm some snobby picky eater. UGH what do i do? :(