I think it's because a causal-effect kind of thing. She probably would be able to make it, but it would not do her anything because she will be consuming the life force that she already extract of herself to make the painting. Now... if she give the cure to another art disease affected person, it would work for them because it would be made with "new" life energy.
This is only a theory of my, because it would fit the theme and ambiance of the novel.

my goofy ass wondering why cant she just paint a cure for her or maybe a potion that could prolong her life (/TДT)/ i feel sad after reading the spoilers:(