"There hasn't been a moment since this tragedy happened that I haven't thought about your family," the star athlete said as Steenkamp's mother, June, looked straight at him in the courtroom, stone-faced.
"I wake up every morning and you're the first people I think of, the first people I pray for. ... I was simply trying to protect Reeva. I can promise that when she went to bed that night she felt loved," Pistorius said after asking for permission to make the apology at the start of his testimony.
Prosecutors allege Pistorius, 27, murdered the 29-year-old model with premeditation by shooting her in the head, arm and hip after an argument and have sought to paint him as a hothead with an inflated sense of entitlement and an obsession with firearms.
Attempting to counter that, defense lawyer Barry Roux led Pistorius through an account of his life, with Pistorius describing some of the hardships he faced after having his lower legs amputated as a baby, the positive role of his mother, Sheila, and his grief when she died when Pistorius was a teenager. Pistorius also spoke about the sacrifices he had made for his athletics achievements, his work with charity and how religion was important to him.
The accounts contrasted with that of prosecutors who, through witness testimony, have painted a dramatically different picture of Pistorius, a man they say was often angry, who cheated on a former girlfriend and who shot a gun out of a moving car in 2012 after an altercation with police and then laughed about it.
Pistorius said he has been taking medication since the week after he killed Steenkamp and has trouble sleeping. He described one night when he went to hide in a closet after waking up in "a panic."
"I climbed into a cupboard and I phoned my sister to come and sit by me, which she did for a while," Pistorius said.
His testimony on day 17 of his trial in Pretoria came on the same day his defense opened its case. Legal experts said it was crucial to his case that he testify to explain why he shot Steenkamp. Pistorius faces 25 years to life in prison if convicted of premeditated murder for Steenkamp's killing.
His voice broke again and he struggled to speak when he described how Steenkamp was "a blessing" in his life. Yet in cellphone messages revealed by the prosecution, Steenkamp had once said that Pistorius scared her.
Apart from his emotional apology at the start, Pistorius didn't directly address the killing of Steenkamp. He will return Tuesday to continue testifying after the judge granted an adjournment because she said Pistorius looked "exhausted." Pistorius said he hadn't slept the night before.
"I'm just very tired at the moment. ... I think it's a lot of things going through my mind," Pistorius said. "The weight of this is extremely overbearing."
Pistorius' testimony also addressed previous instances of crime that affected the runner and how he felt vulnerable because of them, an attempt to explain his claim that he reacted to what he thought was a dangerous intruder in his bathroom by shooting his 9 mm pistol.
He described how his family had "security concerns" when he was young and his mother slept with a gun under a pillow on her bed.
Pistorius said his family had been targeted by criminals over the years, citing incidents of house break-ins and carjackings, and said he had sometimes been followed by unidentified people while driving home. Pistorius also referred to an incident in which he was allegedly assaulted at a social function in late 2012 and had to have stitches on the back of his head.
At the start, Pistorius spoke in a soft, shaky voice while making his apology and describing what he said was his fragile state. Later, he grew more settled and confident as Roux led him through the questions about his childhood, his family, his track career and how he overcame his disability to run at top track meets.
Pistorius's life story is one that impressed many people around the world, before he killed Steenkamp.
He was also asked by Roux to talk about a 2009 boat crash when he suffered serious facial injuries. He said the accident had a "massive impact," and that it made him become fearful, withdrawn, more vigilant about personal safety and more focused on his running.
Prosecutors have provided a contrasting picture of Pistorius, with evidence indicating that he had been reckless with firearms in public and asked a friend to take the blame for him when a gun fired under a table in a busy restaurant while he was handling it weeks before he killed Steenkamp.