There will be times when none of you can do anything right for her and that's ok. It's just part and parcel of what is a crappy disease to get.
Believe it or not when we heard sexual things might be pushed to the sidelines we made the most of the time we had in the short time before treatment. We even went to a sex aids shop (first time in our entire lives) and had an absolute ball, laughing our heads off when we got home and compared notes. I remember that day with fondness as it was totally out of character for us but it was fun!