The 'Roar' hitmaker was apparently devastated when her two-year romance to the singer ended in February amid claims that he had cheated on her, reported Contactmusic.
"Katy was devastated when they broke up. They had talked about marriage and she seemed to think he was a changed man, so she is beyond gutted.
"But Katy is a firm believer in alternative therapies and she wants to wipe him from her life for good, so she decided to see a hypnotherapist. She wants to retrain her brain to prevent John coming back. She doesn't want to find him attractive anymore," a source said.