I agree that being miserable has no sense. But it's not something very easy to overcome. She had a good mood at first, but seeing death before your eyes, and being confined to home and dependent on others gives you little to be happy about.

By the way, you are not more likely to get better if you believe you will. That is a myth. Recovery from cancer has been shown to have no correlation to the patient's attitude and outlook. And in any case, the chances of survival with metastatic melanoma are very very poor. Nothing but surgery really cures melanoma, and you can't do surgery on metastasis in the brain and lungs.