yep, it is really hard, but it does eventually become second nature.
dd1 is gf/cf (and dd2 as well - partly due to caution on our part, but also so that they eat the same thing which makes life a lot easier)
we ALWAYS go self catering. We only stay with people if for a short time (ie overnight) as can then arrive with food made, not take over kitchens etc (well, only to heat food) and breakfast is easy - cereal and long life rice milk.
we have just returned form a 10 day trip to the wilds of Scotland
it was a nightmare in the planning, as had to take enough food for 10 days for both dds (things like cereal and milk, bread as didn't want to have to make it, snacks etc) - their food took up most of the packing room (we have a Viano, so no small car, but it was still a struggle)
we recently bought the bus as it suits us - it has a table so we can sit around and eat together on days out, we have bought a fridge so that car trips are easier, and in the process of getting a 12v cooker - I kid you not! - so that Ican heat up the girls' meals. simpler froma ross contamination issue, and also due to fussiness on dd1's part - even if I were to find something she could eat when out, then she prob wouldn't touch it!
As Peachy said, take a snack bag wherever you go - rice cakes, larabars are great as packed with nutrition and filling, fruits always an option (assuming willing child!). I also use some frezze dried fruits from a company called Just tomatoes - fantastic, and the girls love them - a whole range of crunchy dried fruits and veg - dd1 loves anything crunchy, so they work for her, and are real food! Bonus!
On the restaurant fron, if we do go, i take dd1's food in a thermos, and serve her with that (plus her usual range of snacks) - we hardly ever have to explain, but I am quite happy to explain the allergies issue to staff if they ask. I have found as long as we are eating as well, no one minds.
Once you are into the swing of it, it does become easier, but it still takes a lot of planning. We are off to Australia for our holiday this summer, and i am trying t get my head around enough food for 24 hours without cooking facilities, and which I can get through security, and that dd1 will eat!