This song is about breaking up with Taylor Lautner and how she's sorry for breaking her heart. He gave her all his love and all she did was a goodbye. I think she was falling in love with John Mayer at this point because in her song Superman(about John) she said she wished the flowers and cards were from him instead(referring to "you gave me roses and I left them there to die") plus she wanted John to wait for her relationship with Taylor was done so she could finally be with him instead.
So this song is saying sorry to Taylor.