I don't know what to do anymore?

OK... I'll try to cut the long story short. Basically i am 15, last year my mother went through cancer. Because she was only 36 at the time, the cancer was extremely aggressive. Even though she caught it in a rather early stage, she still needed to have a mastectomy, 10 weeks of chemotherapy, radiotherapy, and hormone treatment for 5 years. But everything that could possibly go wrong did. First of all my mother has always been very self-conscious about her weight. So she has always been very skinny (no one can convince her that she's not fat [she's only 49kg!]), so when they needed extra tissue for the mastectomy she had no fat. So they used the muscles on her back to replace the lost tissue on her front. The result: my mum could no longer lift her arms above her head, and everything became painful. I never realized how much we use our back muscles, but she can't even sit up without help, nor can she ever sleep. Another thing that went wrong is that while going through chemo, her weight dropped as well as her white blood cell count. In the end she needed a blood transfusion. However by the 7th week into chemo she quit. It's not that she gave up really, the doctor allowed it. She was extremely malnourished, her white blood cell count was continuing to drop, not to mention the mental and physical pain was pushing my mothers mental state out the window. The doctor even said that her dosage of chemotherapy was stronger than average. Every day i would listen to her, and remind her that one day we'll look back on this as just a memory. That seemed to get her through each day. She never went through with the radiotherapy or hormone treatment, so that heightened the risk of getting cancer again. That was last year. Throughout this year she has had 4hrs of sleep or less every night, because she is a very light and restless sleeper, not to mention her back pain has not gone away. My mother is very emotional and stresses a lot. She tends to care too much about everything and everyone. This causes her to stress A LOT. My mother has dealt with depression her whole life, and has tried to commit suicide from drug overdose a number of times. Cancer has pushed her back into depression and i fear the worst. My brother and father know about this, but not to the extent that it is. My mother is very good at hiding her feelings, and tries to keep positive. She says that i have been the one always there for her, but i don't think it's enough anymore. She needs to live for herself, not me. She says she has planned the way to end her life, but she is trying her best to keep positive, and think about those that care about her. It is the only reason why she is still alive. To make things worse, she told me that she thinks that she has bowel Cancer. I looked it up, and she has almost all the symptoms. She said that if she has it, she won't do anything about it. I know how much she wants to give up on life. I don't want to lose her, i know this is selfish, and i feel like i have tried everything. I just don't know what i can do. I feel hopeless. So sorry for the super long explanation, but if you took the time to read it, i am grateful.