Rather than saying, "I can't, I'm in pain", I want people to say "I'm in pain, but how can I make this work? How can I do it anyway? How can I prove my pain is not the boss of me?" This is true not only of pain, but of any symptom associated with illness. In this way, it is possible to achieve goals, even with the horrible challenges illness poses.
I'm not saying that it is easy to do this, or that by being told to see the positive, you'll automatically be able to. What I'm saying is that it is worth the effort of training yourself to see the positive, and strive towards your goals. Although it is challenging, it is also thoroughly rewarding. Try to find one positive thing in each day, and write it down, or share it in the comments. You'll be amazed by the difference a positive attitude can make. It's not a cure, but it can make illness easier to live with.