It is very difficult, being the one going through treatments, etc. It is a roller coaster ride, and one really doesn't know how it will "end"...Your mother may need a little help from the doctor to fight her depression...I would strongly suggest that you allow her to feel sad and of course, depressed, she has the right...on the other hand if she doesn't have any days of laughter, positive thinking, and feels "doomed" then she needs help. DO not disregard her right to feel sad, but if she is like that on a constant basis...it needs looking into. God Bless you and your mother in this difficult journey.
Peace, Angie