From my point of view, the key to a disabled child’s happiness is simply treating the child the same way you would treat any child. If you treat her like a bother, like a burden, it won’t matter how much YOU think she doesn’t understand, she’ll pick up on that. Any child will feel and act in the same manner in which they are treated. And if it takes that much more effort to convey your feelings to them, make the effort.
It takes no effort to make a child feel like a burden. So work that much harder to make them feel special. If she could walk, talk, do everything on her own, you would have happily made that effort anyway.