After witnessing the narrator's emotional collapse through the story's end, do you conclude that she displays herself as legitimately psychotic, or do you think the “treatment” by her physician-husband has made her so?

Charlotte Perkins Gilman has included many aspects in her own life, mirroring the narrator; namely, PPD. At the time when "The Yellow Wallpaper" was written, 1892, women were still seen as delicate creatures. Perkins Gilman herself was given a "rest cure" by Dr. S. Weir Mitchell. The narrator's treatment prescribed by her husband parallels Perkins Gilman's "cure." In fact, Dr. Mitchell is even mentioned in the text itself.
While the narrator does suffer from mental illness, it is only made worse by her husband's idea of a cure. From the start of the short story, the narrator is locked away in a room with a bolted down bed, "rings and things" on the walls, and a barred window. John treats his wife as a childish mental patient, as he calls her juvenile pet names, rather than a spouse. Indeed, John's sister, Jennie, even watches over the narrator, encouraging her not to do anything too strenuous, including writing.
The narrator is isolated, alone with her own thoughts and the "horrid" yellow wallpaper. As one does when one is bored, she traces the patterns in the wallpaper. These patterns soon take form, a hallucination brought on by her illness. The madness in the wallpaper is a reflection of her own spiral into insanity. The woman who shakes the wallpaper is the projection of narrator herself, trapped, alone, and striving to escape. Clearly at this point of the story, the rest cure John prescribed is anything but effective.
As the narrator continues to succumb into her madness, we see less and less of John, or even Jennie; it's simply the narrator and the wallpaper. The woman in the wallpaper needs freedom, just as the narrator needs freedom from her illness. The climax of the story where we see the complete breakdown of the narrator as she "frees" the woman in the wallpaper by tearing it down, serves as the peak state of her psychosis. She finds a rope and throws away the key to her room. John comes and faints at the sight of her. The narrator questions why he should faint, giving the assumption that the narrator's appearance reflects her inner turmoil or even that she committed suicide and tells the end of her story from a voyeuristic perspective.
So while the narrator does suffer from a psychotic breakdown, her lack of proper treatment ultimately led to her assumed suicide. One cannot completely blame her husband; however, if we take into consideration Perkins Gilman's own life, perhaps her implied intention was to showcase the misunderstanding of her own husband and doctors at the time. Perkins Gilman "escaped" from her own husband and baby to travel west. She got her freedom just as the narrator did.

The protagonist in Charlotte Perkins Gilman's "The Yellow Wallpaper" does experience an emotional collapse at the end of the short story. That said, to determine if her breakdown is the result of psychosis or treatment by her husband, the reader must look at how her breakdown "speaks" to him or her individually. This type of answer is subjective.
Literary critics have speculated that the protagonist suffers from postpartum depression. Her husband, a doctor, has "prescribed" the treatment plan he believes to be the best for her: isolation and time to relax. Unfortunately, many people do not do well in isolation. Given that people are social creatures, humans need social interaction. Isolation and lack of social interaction may cause one to lose one's grasp on reality. Due to the protagonist's isolation, one could infer that she went insane (when compounded with her PPD).
Therefore, one could argue that her undiagnosed PPD could have led to her breakdown. One could also argue that her husband's treatment, or lack of treatment, led to her emotional collapse. In the end, a solid argument can be made for either point.