Taylor Swift song,maeaning

Today's Taylor Swift song, the meaning of  "Back to December", released in her "Speak Now" album.

This song was a turning point for Taylor Swift as it was an apology rather than a song about an ex boyfriend. It was a song about how she was wrong. I like to compare this song to "Dear John". Taylor Swift admits that she was in the wrong and acted like some of the people that she writes songs about, "You gave me roses and I left them there to die".
                         
                          
          


Taylor says, “’Back to December’ is a song that addresses a first for me, in that I’ve never apologized to someone in a song before. This is about a person who was incredible to me, just perfect to me in a relationship, and I was really careless with him. So this is a song full of words that I would say to him.”

Who was this song about?