The final part of The Bell Jar describes Esther waiting for her final interview with the doctors. She is anxious but ready to leave the hospital. This final part of the book suggests that Esther, after her departure, will lead a life marked always with the fear that her madness could overwhelm her once more. Indeed, she acknowledges that she is "scared to death," and she says that all she can see ahead of her is "question marks." These "question marks" imply that Esther is still not convinced that she will be able to cope with life outside of the hospital.
However, the fact that Esther has doubts, and seems to realize that her release from the hospital does not necessarily mean that she is or will ever be completely cured, also suggests that she is here being entirely rational. It would be irrational of Esther to think that her release represented a definitive end to her issues with her mental health. In this sense, Esther's doubts and fears suggest that she will be able to cope with life after her departure, because she is rational enough to be wary of any signs that her madness is returning, and will thus be more prepared than otherwise to cope with any relapse.
In terms of her agency as a woman, the final lines of the book suggest that Esther is able to exercise a degree of control over her own life. She is no longer passive. When she enters the room where the doctors are waiting for her, Esther says that she "guid[ed] [her]self." This phrase is important. It suggests that Esther has a degree of autonomy. She is her own guide, and is no longer being passively pushed this way or that by people and forces outside of herself. She describes this ability to guide herself as "a magical thread." The connotations of the word "magical" suggest that this newfound autonomy is positive, thrilling and empowering.