Emma and you had a relationship but she chose her career over you and she left you with some other dude so you proceed to finish College and go on with your career but you still love her and there was no hope that you'll be back with her because she's soon-to-be-married

At night, You had been Invited to sing at a restaurant and when you were singing. You look around and you see Emma with her fiancè as you look away in disbelief as she sings her song and it refers to Emma and her Fiancè.