Rebekah,
I have one word for you: YOGA
And if that works for you, then I have another recommendation: Tae Kwon Do
Yoga will let you go at your own pace and feel how your body and lungs react to certain breathing methods. Sometimes I have problems regulating my breathing the way I'd like (for example, taking slow, deep breaths throughout the day) but when I'm in yoga class, all that seems to disappear. I've heard that when a CF patient is working out, their lungs functions increase and what seems difficult to impossible when you wake up in the morning can be easy when you have let your body and lungs warm up properly.
Either way, physical activity and workouts (even weights) help a great deal. So start slow and you will be amazed at what your body is capable of.
Peter