Hi Mom,
So sorry to hear about your daughter. I can imagine how stressful it is for the two of you.
My situation was similar, I fell into the high 60s/70 baseline in my late-teens/early twenties. Not sure why, I always figured it was a combo of things. I was juggling college, a part-time job, and still trying to live the "college lifestyle" and go to parties/socialize etc. Although I was still quite compliant (I'd say 80-90%), I think I was just wearing myself down something awful. I really wanted to prove to myself I could do it and have it all, and live a "normal" life like all my friends. The truth is, it probably was just too much.
For a long time I also attributed it to natural progression of the disease. Now, while that could still be true, it doesn't look as likely due to the fact that here I sit at age 34 and still have baseline lung function in the mid to high 60s. So I really hope hers levels off too. Such a tricky/unpredictable disease, the unknowns and the what-ifs are scary. I went through a period of time where CF is all I thought about too, so I completely understand. I think it's when the progression smacks you in the face that really makes your brain go wild.
Sorry, I'm just rambling, but can really identify with her situation. My words of encouragement would be that I graduated from college, worked in social work, got married, and now have a 3 year old daughter. Just because she is currently in a funk, doesn't mean that she will certainly continue to lose at the rate she is now. I'd maybe see if it is possible that she is doing too much, and having a hard time balancing it all. I know it is hard to give some things up, and slow down a bit, but if it means your long-term health it could be worth looking into. Like I said, at the time I didn't realize that I was exhausting myself, or realize that exhaustion alone can have dire consequences with lung function. So it may be something to look at, even if you or she don't currently think it's a problem.
Another idea that is a little wild (forgive me), but could be worth a mention is hormones. Researchers are learning more and more about the effect that female hormones play with lung function. The more I have read about it recently, I wonder if that could have been the reason why my problems started in my teens/early twenties. I have a couple friends with CF that have switched to continuous (no period) birth control for this reason. Someone posted an interesting article about it on FB recently, if I can find it I'll try to link it for you.
One other thing that I hesitate to admit, is that i did drink in college. I wouldn't say anymore than "average" college kids, but I did binge drink on a weekend fairly frequently. Alcohol is SUPER drying and bad for the lungs. After I got older and wiser, if I had a drink or two with friends I would have a full glass of water for every alcoholic beverage I drank. Also was sure to hydrate before going to bed, and the next day. Now I haven't drank at all for years, and I think about how bad the binge drinking was for my lungs. Anyways, I'm certainly not implying your daughter drinks, just wanted to mention it in case it could be helpful.
Well, I think I've mentioned all my ideas. I hope I've been even the least bit helpful!
Take care, and good luck to you and your daughter. She is very lucky to have you, some with CF don't have anyone who understands!
Autumn 34 w/CF