I think this is about a guy who was best friends with a girl who he fell in love with. Now, she's getting married to someone else, and he never got the chance to tell her about how he felt about her. When it says, "You left me", I think this means she won't be with him like old times, cause she has a husband to share her feelings with. He regrets he could never build up the courage to tell her he loved her, and now it's too late.