Actually, it sounds to me like she's being sincere. She's realized that he's not Mr. Right, so she's leaving. That first verse says it all. She saying she game him plenty of time to "find himself," so now she's leaving and not looking back ("Got a lead foot down on my accelerator and the rear view mirror torn off 'cause I ain't never lookin' back.").
Actually, it sounds to me like she's being sincere. She's realized that he's not Mr. Right, so she's leaving. That first verse says it all. She saying she game him plenty of time to "find himself," so now she's leaving and not looking back ("Got a lead foot down on my accelerator and the rear view mirror torn off 'cause I ain't never lookin' back.").