Buy her a dog or a cat that she can cuddle up with. One from the humane society. Hire someone to be with her while you are gone - two to three times a week. Take that time to go running or some kind of challenging exercise.
It is very important that you keep your mental and physical health up as the caregiver. But you have to get up and get going! And stop feeling sorry for yourself, even if you are justified.
Another cancer survivor