Avril Lavigne destroyed her career by releasing "Girlfriend"
The song was a mega-smash, but most of her huge fan base jumped ship when "The Best Damn Thing" was released. The decline in quality, just to be more "upbeat" and "happy".
Instead of catering to the fans that made her the superstar she was, she decided to try to appeal to the current crop of teenagers who barely knew her, and therefore, flopped ever since.