Taylor Swift's Dear John

Long were the nights when my days once revolved around you
Counting my footsteps praying the floor won't fall through again
And my mother accused me of losing my mind
But I swore I was fine

You paint me a blue sky and go back and turn it to rain
And I lived in your chess game, but you changed the rules everyday
Wondering which version of you I might get on the phone tonight
Well, I stopped picking up and this song is to let you know why

Dear John, I see it all now that you're gone
Don't think I was too young to be messed with?
The girl in the dress cried the whole way home
I should've known

Well, maybe it's just me and my blind optimism to blame
Or maybe it's you and your sick need to give love then take it away
And you'll add my name to your long list of traitors who don't understand
And I'll look back and regret how I ignored when they said run as fast as you can

Dear John, I see it all now that you're gone
[From: http://www.elyrics.net/read/t/taylor-swift-lyrics/dear-john-lyrics.html ]
Don't you think I was too young to be messed with?
The girl in the dress cried the whole way home

Dear John, I see it all now it was wrong
Don't you think nineteen's too young to be played with?
Your dark twisted games when I loved you so
I should've known

You are an expert at sorry and keeping lines blurry
Never impressed by me acing your tests
All the girls that you've run dry have tired, lifeless eyes
'Cause you've burn them out

But I took your matches before fire could catch me
So don't look now
I'm shining like fireworks over
Your sad, empty town

Dear John, I see it all now that you're gone
Don't you think I was too young to be messed with?
The girl in the dress cried the whole way home

Dear John, I see it all now it was wrong
Don't you think nineteen's too young to be played with?
The girl in the dress wrote you a song
You should've known

You should've known
Don't you think I was too young?
You should've known

I remember hearing somewhere that Swift's songs are very autobiographical. So who is this 'Dear John'? Here is a common theory.