Taxi firm boss Nasser Rezaie, 63, of Petersfield in Chelmsford, is accused of murdering Allan Frampton in May.
Mr Rezaie told Chelmsford Crown Court he noticed his wife's behaviour had changed when he returned from a trip to Iran in 2013.
He denies murder but accepts killing the 60-year-old divorced grandfather.
He argues his actions were a result of losing control and were not premeditated.
Mr Rezie told the jury his wife Jane had started wearing false eyelashes and making regular trips to the hairdressers when he returned from Tehran.
She admitted the 10-month affair with Mr Frampton, who rented a car workshop unit from the couple's taxi firm, after a confrontation with Mr Rezaie in June last year, he said.
The court previously heard Mr Rezaie made threats to kill Mr Frampton over the phone, which were recorded by Mrs Rezaie.
The jury was told he repeatedly ran over Mr Frampton with his silver Mercedes in Osea Way in Chemsford 11 months later.
Mr Frampton, a mechanic, died at the scene from multiple injuries.
Mr Rezaie broke down in tears as he told the court how his relationship with his children suffered following the breakdown of his marriage.
He said they were angry with him and admitted he had written "terrible things" in a series of Facebook posts in June and July 2014.
He also described how his business, Ali's Taxis, had suffered after the breakdown of his relationship with his wife, and alleged she had run up debts of Â£176,000 which he had to pay off.
The trial continues.
A man accused of murdering his love rival has told a court how he "collapsed" after his wife admitted she had been having an affair.