To put it bluntly it's a slow suffocation. I had a friend who passed away last year from cf. She was hooked up to all the machines and was pretty doped up on pain killers, so I don't think she really felt or knew what was going on. Myself I got lucky and got transplanted before I got that bad. But you are young ( i assume from the sub forum of teens and young people) so do not think of this, nor should you ever think of giving up, its not an option. You should be focused on keeping your lungs as healthy as you can and living life to its fullest. With all the advances in medicine and treatments you should live a good long life.
I've been pretty close and all I can say is that as your O2 level drops your brain starts to shut down, but it is pretty close to drowning. I've heard that a way to get an idea what it's like to have lung problems is to try breathing through a straw while you exercise but I never had to try it, as my lung function is normally in the low 20's.