Why does the narrator begin wearing a rubber band on her wrist in the novel, All Fours?

In Part 2, the rubber band the narrator starts wearing on her wrist is symbolic of punishment. After parting with Davey, the narrator does not know how to deal with her heartbreak. Every time she thinks of Davey, she therefore snaps the rubber band on her wrist. She believes that she is a bad person and that the only way to get over Davey is to punish herself.