This song is called Hitoiro and it's by Mika Nakashima.
It was the OST of NANA II. The lyrics mean a lot to me.

"Another single flower petal is torn off
And I don’t have the power to stop the wind
As you break down crying on the far shore
This is the least I can give you in parting
You can forget the promise we made that day
How can a love that was just a step too late
Tear everything apart like this?
Once again the stardust flows
And I don’t have the power to stop time
Say a prayer, you only need one
It’s okay to wish for your own happiness
We allowed pain of the same color
While condemning mistakes of a different color
We lit fires of the same color
While hiding doors of a different color
Now a faded flower scatters for you
Now a faded star scatters for you
In your dreams, good night
How can a spoonful of love that got left out
Ruin everything like this?
On the street corner, it looks like another squall will hit
And I don’t have an umbrella to guard my wounds
Find the exit, you only need one
It’s okay to search for your own happiness
We beat rhythms of the same color
While silencing footsteps of a different color
We painted scenery of the same color
While shutting away seasons of a different color
Now a faded night scatters for you
Now a colored morning comes for you
In your dreams, good night
Sweet dreams, baby
Sweet dreams, baby
If someday we meet somewhere again
Will we be able to try again? Is there anything left?
Time’s about to overtake us, but we can’t reset it
Don’t cry, kindness isn’t weakness
We allowed pain of the same color
While condemning mistakes of a different color
We lit fires of the same color
While hiding doors of a different color
Now a colored flower blooms for you
Now a colored star falls for you
In your dreams, good night
Sweet dreams, baby
Sweet dreams, baby"