Songfacts®:

• This slow-building anthemic track is the lead single from What's Inside: Songs From Waitress, an album featuring songs from the Broadway musical Waitress, for which Bareilles wrote both the music and lyrics. Waitress is a musical adaptation of the 2007 film of the same name.
• Bareilles explained the story behind this song during a Reddit AMA: "I wrote it for the scene in the movie where the lead character, Jenna, feels like she's lost all hope of saving herself from her terrible situation, and she's reflecting on who she thought she would have become," she said. "I found myself in her story and it was the first song for the show."
• Although the song addresses a specific scenario about an abused pregnant woman coming to terms with the life she lost, the emotional weight of the lyrics connects listeners from all walks of life. Tiffany Mann was a struggling actress making ends meet by waitressing and singing at Ellen's Stardust Diner in New York City when her version of the tune hit social media and led to a role in the production of Waitress. "In that video, I wasn't singing 'cause it was my job," she told The New York Times. "I was singing my way through heartache."
  
  Broadway stars Heather Headley, Ben Platt, and Kathryn Gallagher also offered their heartfelt takes, but one of the most popular covers came from an unlikely source. Adrian Matthew, a 14-year-old student from Pennsylvania whose cover went viral in 2018, said he had no trouble relating to the song. "You don't have to be a middle-aged woman who is pregnant and abused - you can still feel it," he explained. "The way I relate to it - there's been school bullies, or people making fun of me. I thought going from middle school to high school was going to be so easy, and I'd make so many friends, and then you get there and it's the opposite. The song made me feel that."
• Speaking to Time Out, Bareilles revealed that she embarked on the Waitress project as she needed to do something different. "I was burnt out and tired of the pop artist cycle," the singer-songwriter explained of pressure placed on her to write an album, record it, tour it, and then start all over again.
  
  Following a lunch with director Diane Paulus, Bareilles went home and watched the Waitress movie. "Within a couple of weeks of just chewing on this idea I wrote She Used to Be Mine," she recalled. "(It) was the first song I wrote for the show."
• Sara Bareilles grew up loving musical theatre and performed it as a child, but she never seriously imagined herself writing a complete theater score until the Waitress opportunity arose.
• "She Used to Be Mine" was personal for Bareilles. While writing it, she had recently moved to New York after 15 years in Los Angeles, leaving behind her home, band, boyfriend, and manager. She connected her own feeling of not recognizing herself with Jenna's struggle to reconcile who she expected to become with who she was.