How is last leaf related to the proverb 'the diseased mind is harmful than the disease itself?

In O. Henry's "The Last Leaf," the character Johnsy suffers from despair and hopelessness due to her illness, reflecting the idea that a diseased mind can be more detrimental than the physical disease itself. Her fixation on the falling leaves symbolizes her deteriorating mental state, as she believes her life is tied to the last leaf on the vine. It is only when she sees the last leaf still clinging to the vine, a selfless act by Behrman, that she finds hope and the will to live. This illustrates that a positive mindset can be a powerful remedy, emphasizing that mental resilience can often be more crucial than the physical ailment itself.