The Life and Loves of a She-Devil (1986)

Ruth Patchett has it all: a large, warty body, a standard suburban home, a couple of unruly children, a dog, a cat and a guinea pig. She also has Bobbo, her unfaithful accountant husband who resents her very existence. Bobbo wants, and is wanted by, romance novelist Mary Fisher, who lives in a lighthouse by the sea. When Bobo leaves Ruth for Mary, Ruth decides that Mary doesn't know the first thing about love, and she's gonna teach her.