Stacey, I don't have CF, don't know anyone with it (I was just getting myself educated when I saw this), so I'm coming at this from a different direction. I think of what it was like to watch a friend die, dwindling away to a mere shadow of his former vital self.
You can't have light without shadows.
Sometimes, the shadows make the light seem all the brighter, and show us things we wouldn't otherwise notice. Life is never so bittersweet as when you are enjoying a pleasure that you may not be given the gift of again. Dawn, a rain shower, the smell of woodsmoke, the company of a loved one, the sight of a person who is trying even if they are failing...the imminence of Death makes us appreciate Life in a way that nothing else can. Until you have stared Death in the face close up, you don't really know Life, except shallowly, as a child does. And this gift, the priceless gift of appreciation, is something that you and your friend can give to each other. And if you do this thing, I promise you this; in the end, you will consider it to have been worth doing.
Yes, the dark is very dark, and sometimes very cold. I won't deny grief is a heartache, and one that time will heal, but never without a scar. Death is ugly; but it can also be inspiring, and educational, and even edifying, because it is life lived close to the edge, where reality loses all disguises.
The most important thing any of us will ever do is be an example; and few of us get to choose what sort of thing we become an example of. But whatever we are, whatever situation we find ourselves in, to care for another human being regardless, to be honestly ourselves with all our vulnerabilities and human failings, and to show love for another - that is something that teaches the whole world what a good example is.
Amidst the darkness of grief, you learn that the only true light comes from the human heart; and forever more, you remember, and can see it.
So my advice to you after all these fine words is, try. It's only human to be afraid to fail, as your friends are; but you will at the very least grow as a human being, and be able to be proud that at least you tried.