The song - which will appear on her forthcoming album Speak Now - allowed the singer to find closure on a difficult relationship during which she acted 'carelessly', reports MTV News.
"Well, I've always sort of written songs about situations in life, things that needed to be said," she explained.
"I write songs about people who deserve to have songs written about them, and whatever they need to hear - whatever is the right thing to say to that person - ends up being said."
She continued: "I've never felt the need to apologise in a song before. But in the last two years I've experienced a lot, [including] a lot of different kinds of learning lessons.
"And sometimes you learn a lesson too late and at that point you need to apologise because you were careless."
Speak Now will hit stores on October 25.