A few days ago I was the love of your life, and suddenly, I became just somebody that you used to know. That feels so rough!
This is a song by Gotye. In the original song, it's only a man and a woman singing, so all the parts are sung by Gotye except the woman's part, who is sung by Kimbra. So we have to take this song as a conversation:
The man complains about the way she broke up. She completely cut off and disappeared and now treats him like she doesn't know him. And then she complains about how hard it was to live with him, and explained that when they broke up, he told her that he could just "let it go" (forget about it) no problem, so she's acting accordingly. But then the man replies and says once again that her reaction was too harsh, too radical, and that hurts.