The changes in the narrator's feelings about the wallpaper reveal that her mental condition has deteriorated.
The narrator's progressive symptoms also reveal signs of psychosis. Psychosis isn't a disease but is usually evidence of a schizophrenic spectrum disorder.
Signs of psychosis include obsessive thoughts that prevail (despite counseling), withdrawal from family and friends, hallucinations, and a decline in self-care. The narrator has experienced all of these. For example, she tells us that her baby makes her nervous; she cannot be with him. As time goes by, she also sees shapes inhabiting the wallpaper.
Of course, no one else sees the shapes except her. A clear sign of the narrator's psychosis is her insistence that there are beings or "things" in the wallpaper known only to her. She is also adamant that no one else will ever see the "things" she sees in the dreaded wallpaper.
To the narrator, the wallpaper is inhabited by many configurations of a sinister shape. That shape, she maintains, is of a woman stooping down and creeping about. Sometimes, she thinks that the shapes are of many women, instead. Later, she maintains that the yellow wallpaper gives off a peculiar smell. So, the narrator is seeing, hearing, and smelling things that others don't. This is a classic sign of psychosis.
So, the changes in the narrator's feelings about the wallpaper reveal that her mental condition has deteriorated and that psychosis has set in.