Depression and coping with secondary cancer

My Mother has cancer and since being diagnosed March 2021 her mental health has got worse month by month. She is taking two types of anti depressants. She has had two long sessions of counselling. None of this has helped. she lost her husband(my Dad) in August and this is an artful thing to bear. We have days where she says she doesn’t want to live anymore. She feels a burden to us. She has told my sister she has tablets stored. We really don’t know how to cope with this. It was so hard for our Dad being with her every day and observing the depression. Since Dad has passed away we cannot spend the amount of hours with her that Dad did. We do all we can to support her, cleaning, ironing, washing, making food. Encouraging her to come out for a drive. All her markers have reduced and the oncologist doesn’t want to see her until next February. Can somebody please offer us advice, or tell us where to get some help.