What is she used to be mine about?

"She Used to Be Mine" is a song written and performed by American singer-songwriter Sara Bareilles. It was released as the lead single from her fifth studio album, "What's Inside: Songs from Waitress," in September 2015. The song features Bareilles' trademark heartfelt lyrics and piano-driven melody, showcasing her powerful vocals and emotional range. It has received critical acclaim for its raw and honest portrayal of a woman grappling with regret and longing for the person she used to be. The song gained additional popularity after being featured in the hit Broadway musical "Waitress," for which Bareilles wrote the music and lyrics.