Derek Rose, 33, went out with Ecclestone, 28, when she was 17, before they split up in Aug 2002.
In Nov 2011 — having already sold a story about her for £10,000 — Rose orchestrated a blackmail letter claiming he would sell his story to a newspaper helped by co-defendant Jakir Uddin, aged 20, it is alleged.
Rose told London’s Southwark Crown Court that he went out with Ecclestone for nearly two years, describing their break up as “acrimonious”.
Asked by his lawyer, John Coffey QC, whose decision it was to part ways, he answered: “Mine. Because I found out she had been cheating on me.”
Questioned about how he now felt about an “offensive” email he sent to Ecclestone after they broke up, he replied: “Embarrassed, ashamed.
The father of one was asked why he sold his story to the Mail on Sunday in 2002. The court previously heard that he had been disappointed by the angle the article had taken as it had focused more on Ecclestone’s father, Formula One boss Bernie, than her.
Rose told jurors he met Uddin in Aug or Sept 2011 and the pair became friends.
He said that when the media student asked him if he would be the subject of a university project, he agreed and they made a short film in which he talked about his relationship with Ecclestone.
He said he was “shocked” when Uddin later said he wanted to sell the footage to the press. Rose said he told Uddin he “wanted nothing to do with it”.
Both deny blackmail.