Dark Victory (1939) on Netflix

Wealthy young socialite Judith "Judy" Traherne spends her time partying and horse riding. She has many friends but no one really close, except for her cousin Ann, who serves as her companion. But Judy is hiding symptoms which are rapidly becoming worse. For several months she has headaches and memory loss. Then her vision becomes affected. Trying to laugh it off, Judy confides in Ann. But Ann takes it seriously, especially after Judy is injured in a horse riding accident. Dr. Parsons, the family's elderly physician, believes her to be seriously ill. He recommends that she see Dr. Frederick Steele, a neurologist. Judy refuses until she falls down the stairs. Reluctantly, she agrees to at least meet Dr. Steele.