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There are two problems here, depression and cancer. The depression was around before the cancer showed up.
The writer may or may not survive the cancer. Surviving the cancer is certainly a possibility, and not a remote possibility. If she doesn't then welcome to the club; we're all going to die. If she does, then she's in good company.
Depression is a different matter. I haven't had cancer, but I've had the living death of the Black Cloud. Fortunately, there are some great medications available, and if one doesn't work one of the others probably will.
The thing is that a depressed person has to take responsibility for his or her depression. You have to do the things that will contribute to your cure. You have to reverse the downward spiral. You have to socialize when you don't feel like it. You have to go to dinner and a movie when you don't feel like it. You have to go to the doctor and try out the medications. Eventually, you will find something that works.
And if it turns out that The Gods decree that you have to go out, then so be it. The rest of us are right behind you. Best wishes.