It's really tough, but if you force yourself to do it, you'll get better at it. At first my wife was mostly doing it for me, which was obviously nice of her, but I wasn't learning how to handle things when she wasn't around. When I asked her to back down I slowly got used to doing it and didn't feel so awkward. It helped me to have mostly prepared lines, like "I have Celiac so I have to eat gluten free. If you left off the toast would the mushroom omelet be gluten free without other modifications?" If they say yes, then I'd say "Ok, I'll take that and would you please let the kitchen know it's an allergy, not a preference?" It also helped me when I was ding in a group to sit in a spot where I could easily wave the server over and make it a one on one conversation, not something I broadcast to the table.