i had a lot of health problems starting at around 15 -- after that i was always in the hospital a couple times a year even if my lung function would typically bounce back. i didn't stop working, and ended up going to high school, college, law school, and then working for a few years before my transplant. i don't regret a moment of it, and i wouldn't do it any other way if i could do it again, but i also firmly believe that every person (CF or not) has to find his/her own way to contribute and feel part of the world. there is no right path: whether it be a particular job/profession, school, marriage, whatever. the point it to LIVE, to be happy, and hopefully to make some sort of difference. honestly, you're on the right path just by asking these questions.
my observation is that it sounds as though you're feeling a little left out. your dad's comments might have stung a bit b/c you feel that you would LIKE to work (or something) but you also feel held back by your disease. and to be honest, that's the only part of your story that i think you need to be worried about right now: making yourself feel as though you are stronger than CF, and that it can't stop you from really feeling GREAT about yourself. let go of everything else (within reason, of course - i am NOT saying neglect your health), and just take a couple of baby steps. ask yourself if there's something you would like to do that doesn't require a huge commitment right off the back. some good examples: photography (been mentioned), painting or art, writing, or some other sort of personal improvement project. volunteering is great, of course, and if you need a little extra cash there are some awesome jobs that could keep you active without being too stressful. do you enjoy animals? pet sitting or dog walking could be fun. just be careful if you have allergies and i don't recommend taking care of birds. or if you enjoy exercise maybe set a goal in that field. running or walking a 5k in your area would be awesome for your health and your spirits, and would TOTALLY make your family proud. also, lots of places offer short classes in interesting subjects like art, acting, cooking, writing, or other skills. great way to meet people, fun way to get out of the house, and some of the skills can transfer directly to things that will let you help out around the house. trust me, your parents won't complain!
final suggestion: at the very least, try weaning yourself OFF the activities that you feel might be making your lethargic or a little unhappy. set a computer limit (like "i will spend 4 hours a day without even looking at my computer or TV!") and just do ANYTHING else. read, walk, cook, play, write, whatever. try out things that you think you'll enjoy and i bet you'll be right -- after all, you know yourself best.
rock cf foundation has an exercise challenge for people with CF right now, and cystic gal (the blog -- google it) has a CF poetry contest this upcoming month. check out both of those for fun ways to connect with the CF community and set a fun, non-stressful goal for yourself. good luck!!