Nancy - Give yourself a pat on the back! Trying to exercise with physical challenges is hard, but if you take your "baby steps" you'll find it worthwhile. Maybe your brain & lungs can work together to "overcome" the effect of your paralyzed diaphragm? Have you had any PT or OT to help you?
The body is an amazing "machine" and you may find that by pushing a little at a time, your endurance will improve.
I compare it to a person with a brain injury - if one part of brain is damaged, you can challenge another part to take over (at least part of) the function. So it is with my lungs - I have part that is damaged, so I work to strengthen the rest to increase breathing capacity - now I can jog along with my little grandson for a block or more - 3 years ago this month I could barely walk from my little house in Texas to the clubhouse 150 feet away.
Good luck.
Sue