A comment on Disability

I have a friend who is "disabled" who has taught me to think "differently abled"

Perhaps because of this I was struck by this comment of Jennifer Culkin in A Final Arch of Sky.  The book is memoirs of a critical care nurse whose career was ended by MS.  "We tend, as a culture and even as health-care providers, to think of disability in terms of either/or, black or white, can or can't.  When it's largely invisible, when some days you can do things easily, and some days you can do them with a big effort and some days, in spite of bringing every personal resource to bear, you can't do anything at all, what then?  It doesn't feel like disability.  It just feels like failure." 

I thought of my friend, whose disability is not always obvious, and how she has wrapped her mind around the idea that she can do whatever her body allows for that day and counts that as an accomplishment.  She seems to feel no failure.

How does she do it?  How do we keep from labeling our own unsuccessful efforts "failure" when they maybe took all our personal resources in that moment.  Is that failure?  What is failure really?  What is success?