"There hasn't been a moment since this tragedy happened that I haven't thought about your family," the double-amputee star athlete said as he addressed the courtroom and Steenkamp's mother, June, who looked straight at him, 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.
Prosecutors allege the Olympian murdered her 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.
In his testimony, Pistorius also said he is on antidepressant medicine and now has trouble sleeping, and 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.
He spoke in a soft, quavering voice while making his apology and describing what he said was his fragile state. Later, Pistorius grew more settled and confident as defense lawyer Barry Roux led him through questions about his childhood, his athletics 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.
Pistorius also described the positive affect his mother Sheila had on his life after he was born with a congenital condition and had to have his lower legs amputated when he was 11 months old, and the grief he felt at her death when he was a teenager.
He also described how as a family they had "security concerns" and his mother slept with a gun under a pillow on her bed.
He was 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 He said the accident had a "massive impact," and that it made him become fearful, withdrawn, more vigilant about personal safety and more focused on his sporting career. He denied media reports at the time that he had been drinking alcohol before the boat accident.