In a case that grabbed international headlines, the world-famous double amputee, 27, shot Reeva Steenkamp through his toilet door, claiming he mistook her for an intruder at his upmarket Pretoria home.
Prosecutors allege the shooting was premeditated murder and the track legend known as "Blade Runner" will go on trial on the charge on March 3, in what is expected to be one country's highest profile cases.
"The pain and sadness - especially for Reeva's parents, family and friends consumes me with sorrow," said Pistorius, breaking his long silence since he shot dead his model lover -- an aspiring actress.
"The loss of Reeva and the complete trauma of that day, I will carry with me for the rest of my life," he said in the message signed simply "Oscar".
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He also posted a link to the message on Twitter, describing it as "A few words from my heart". It was the first time he has used his official account on the social networking site since February 12, 2013, two days before the shooting.
"We will release balloons in her memory," her uncle Michael Steenkamp told AFP. "They will be red and white, her favourite colours."
The function will be near Cape Town, "away from the city and the concrete jungle", he said, adding her parents and brother would not attend.
Saying that "nothing will ever be the same", Steenkamp's mother June, who lives in Port Elizabeth east of Cape Town, told The Times newspaper that she would attend next month's trial.