How to Deal (movie, 2003)

Mandy Moore stars as Halley, a young hip high school student who's convinced true love doesn't exist based on the crazy relationships around her. Her mother is divorcing her father who is dating a younger woman Halley can't stand. Her crazed sister is planning a wedding but has second thoughts and her best friend has fallen madly in love for the first time leaving Halley to feel even more alone.