What is the name of the song sung by a woman about her X boyfriends new fiance'?

The song you're referring to is likely "Before He Cheats" by Carrie Underwood. In this track, the narrator reflects on her ex-boyfriend's new relationship and expresses her feelings of betrayal and empowerment. Another possibility is "Picture to Burn" by Taylor Swift, where she also addresses an ex's new girlfriend with a sense of defiance.