Teenage book i read in the late 1980s

[Dimonite]

Hi, today i was thinking about a favourite book i had when i was a young teenager. I cant remember the title does anyone know? It was about a young girl called Kathleen who goes to stay with her cousin who she thinks is beautiful (and gets great boyfriends) and she is a shy young skinny girl. She goes to the beach in a long T-shirt as she does not want to anyone to see her, its about the struggles she has with this and also finds out her mum and dad are splitting... does this ring any bells what-so-ever?