How does Mrs. Drover control her fear?

After finding a mysterious letter on her hall table, Mrs. Drover retreats to the bedroom of her abandoned house to read its contents.  In the narration that follows, we learn that Mrs. Drover's normal expression is "one of controlled worry," which suggests that she is already an anxious woman by nature.  As she processes the contents of the letter, she moves from her position with her back to the empty room to a chair against the wall--an obvious sign of paranoia.  Further, Mrs. Drover tries to convince herself that she is "in a mood" and has imagined the letter, but after shutting her eyes and opening them again, she finds the letter is still there.  After locking the bedroom door, she hastily packs her belongings and prepares to--and eventually does--flee the house.  Once out of the house and back in the presence of passersby on the street, Mrs. Drover is convinced that she has successfully escaped a dangerous situation.  Obviously, though, this is not the case, as she presumably comes face to face with her Demon Lover in the taxi.