Reeva Steenkamp and Pistorius sometimes had troubles in their relationship but they sorted them out, were in love and were planning a life together, the double-amputee Olympic athlete told the court.
Prosecutors say Pistorius was often jealous and overbearing in his relationship with Ms Steenkamp and killed her intentionally after a loud argument in the pre-dawn hours of Valentine's Day last year.
Pistorius referred to a phone message Ms Steenkamp sent him in late January 2013 when she said she was sometimes "scared" of him . The prosecution presented the message earlier in the trial as an indicator of Pistorius's threatening behaviour.
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Pistorius said the couple had a disagreement at a social function that day and he was "maybe just being sensitive, insecure or jealous".
He said he apologised to Ms Steenkamp and sent her a message saying: "I want to talk to you. I want to sort this out. ... I'm sorry for the things that I say without thinking."
Then, addressing the judge, he said: "My lady, I think it was a bad day in our relationship."
Defence lawyer Barry Roux asked Pistorius if the unhappiness in their relationship had passed and Pistorius said it had.
Pistorius was being led through questions by Mr Roux, with the defence attempting to counter accusations the runner was a reckless hothead with an obsessive love of firearms and prone to outbursts of anger.