I suppose my question would be: What's the cause of your ED? Have you seen a urologist? Contrary to popular belief, taking a viagra, or levitra or cialis, doesn't give you an instant erection. These drugs are mainly for guys who have vascular damage and need some extra help getting or keeping an erection.
Could you be on any medication that could contribute to your ED? Things like benzodiazepines, narcotics etc. Stress is also a major culprit, as is your very low FEV1 and the CFRD. In the case of CFRD, viagra might help, but with low pulmonary function, I don't think it would make a difference.
I hope you get things worked out. There are other options that are not medications that I have heard of, but those are best discussed with a urologist.