The female lead was previously hurt by a boyfriend who wrote a book about her and ruined her career and the male lead is a singer songwriter. I forget how they meet but she ends up helping him write a song for this 'eccentric' but slightly insane singer (pretty sure her character was inspired by Brittney Spears and Lindsey Lohan). There's a scene where the lead female is helping the lead male write this song and decides he needs to rearrange his apartment, and another where he convinces her to sing the vocals for the demo. It's cliche, so they fall in love. They give the demo to the singer who decides to change it entirely. the female lead wants to tell her it's terrible, the male lead thinks the female lead shouldn't say anything, they fight, she leaves, he becomes a washed up singer doing small gigs. She gets a job working at a hair salon? The male lead shows up to try and make amends but I'm pretty sure she turned him down. He eventually becomes big, he gets a spot singing with the crazy singer. He sings the song that he and the female lead wrote. She is at the concert, hears it, they fall in love again.
That's about it, it's been a while since I saw it. I've tried googling it but all that came up were Once and Begin Again both directed by John Carney. I watched the trailers but I don't think it was either of them. Does anybody know what this movie is?