Neiltheblue is right as far as I can see.
Mum is having an affair with some guy from the top of the estate, he has killed himself leaving a note for her. She can't express her grief at loosing her lover to her family. As such, she is feeling lonely and the family don't know what wrong with her. The 'word means nothing' at all refers to her breaking her word by breaking her marriage vows. Therefore, when your word means nothing you feel lonely.