The song is Every Heart, sung by BOA. It's the English, not the Japanese. And this is not a Gaara/Temari fanfic. In fact, I am very much against that pairing. It's Shika/Temari all the way, peoples!
You Can't Change the Past:
Sasuke pushed her into the river. If only she had known how to swim.
But she was from the Sand, and was not even used to rain. Shikamaru watched her get beaten by the pounding rapids and sharp rocks, unable to pull her out until the calm waters.
If there were many tears falling down,
Every heart would become gentle.
If everybody expresses what they think,
Every heart can be satisfied.
The girl from the Sand had been taken to the hospital in Konoha, and healed. But there were parts of her memory that were blank. She remembered a brother, but that was all. A puppeteer. She knew how to use her weapons. She knew most of the missions she had been on. But so much was missing.
In endlessly repeating time,
We are searching for love,
Because we wanted to become stronger,
We look up to the far away sky.
Once the Sand had once again rejoined forces with the Leaf, Gaara and Kankuro were able to visit their sister. She recognized Kankuro.
Gaara thinks back to a time when he was given a chance.
The two of us meet, smiling here,
Every heart has a habit of receiving its dreams.
Sadness has no effect on us,
Every heart gains happiness when it flies.
Flashback
Gaara sat on a cliff. A young girl approached him, frightened but dignified. "Hey, Gaara," she called. When she got no answer, she approached him, and sat next to him.
She began to sing a song that her mother had taught her; before Gaara had killed her.
In endlessly repeating time,
We know why we are living.
We go through nighttime laughing,
Both of us are mourning, but we walk on.
She finished the song. There was a long silence, until she said, "Mother used to sing that to Kankuro and I. But…you never got to hear it, and so I thought I could come and sing it too you…as you older sister…"
Memories of everything has settled,
This is a warm place to be.
The stars separate us from the future,
We are always so brilliant;
So shine.
Gaara's sand grabbed her and threw her against a boulder, holding her there.
"Ah!"
"Don't ever speak to me. You are not my sister! How could you ever understand me!" Gaara yelled at her, voice full of hate.
The sand slowly let her go, and she fell to her knees, whispering, "I can't…but I tried."
End Flashback
In endlessly repeating time,
We are searching for love,
Because we wanted to become stronger,
We look to the far away sky.
Gaara couldn't blame her for not remembering him. He watched as his sister and brother talked, laughing.
"So it's my own fault," Gaara whispered, turning away. "You can't change the past, but I can change the future."