Hi Jenni,
One thing to remember, as hard as it is to deal with, is that this diagnosis will improve the quality of life for your daughter [and sons, down the track]. Knowing that they have CF will now allow them to get the proper treatments, which will help their health A LOT. Your daughter's weight loss may be helped by enzymes.
One of the first things I'd do is try to find a CF clinic close to you, call them and explain your situation. As well as telling you how you can get an appointment, they will probably be able to recommend a counsellor or social worker to help you with some of the things you're dealing with. A specialised CF doctor will be a lot different to deal with than a regular GP and I think you'll find they are able to answer all your questions that a "normal" doctor could not. At clinics you usually have access to different people like pulmonologists, nutritionists and anyone else that is more specialised for dealing with the more common [and less common] problems with CF. Just as a note, I feel frustrated with most GPs even though they know I have CF.. my CF doctor is so much easier to talk to and understands how different each case can be.
To give you an idea of my situation.. I am 20 years old and live in Australia, and was diagnosed at birth [we have a mandatory genetics screening process for every newborn]. I weighed 5 pounds 13 ounces and also had issues gaining weight. Putting me on digestive enzymes helped a lot and although I've always been pretty skinny, my weight has never been a big problem since I was little. My case of CF is quite mild so I didn't really miss much school, except that the bugs going around always affected me more and for longer periods than most other kids.
With CF, you just have to take every day as it comes. Try not to think about the life expectancy, because it's just not a true indication of the disease. There are people in their 60s and at least one in their 70s with CF. Each case is so very different that all your children may lead totally normal lives, aside from some extra medication and occasional hospital stays. I really believe that a diagnosis is a step forward, even though it's hard to deal with.
I wish you tand your children he best and will keep you in my thoughts.