Mother is diagnosed of terminal illness and she is too negative about it

My mother has been diagnosed with Ovarian Cancer. She went through one chemo so far and she is recovering. Even the doctors are very positive. However, she is **extremely** negative about her situation. She is constantly questioning why Allah is making her go through this pain, not praying/making dua and no matter how much anyone is trying to convince her, we see no change in her state of mind. Her 2nd chemo is coming in two weeks time and before that we want her to think positively and stay hopeful. We are trying to see if we can get professional help but at the same time, I would also like to know: **Question:** Is there any dua/namaz that can effectively change her thinking?