Why did Joan abandon her son in the novel, The Unlikely Pilgrimage of Harold Fry?

Joan was Harold's mother. She left Harold when he was very young and he never got over it. She may have been manic-depressive. She was sometimes very happy and laughed at Harold's jokes and was sometimes so withdrawn that she paid no attention to him at all. She wrote him a note after she left, saying she wasn't cut out to be a mother.