Stacey Flounders, mother of Johnson’s daughter Ayla, said she was forced to make the decision when she discovered she was 10-weeks pregnant soon after the arrest.
“I couldn’t bring another baby into this world with everything going on,” Ms Flounders said.
“It was heartbreaking but I felt I had no choice.”
Johnson is reportedly set to be released from prison later this month, having served half of a six-year sentence for kissing and grooming an underage girl.
Speaking to the Mail on Sunday, Ms Flounders said her life “fell apart” when the footballer appeared in court.
The charges against him related to an incident in the former Sunderland midfielder's Range Rover when he met up with the girl in 2015.
Johnson initially denied two counts of sexual activity with the girl, one count for kissing and one count for grooming a child.
Ms Flounders said she knew he would plead guilty in court for kissing the girl, but did not know he would also plead guilty to grooming his victim.
“It was the moment I realised who Adam really was,” she said.
“I watched the man I had been in love with – the man I wanted to marry – disappear.
“I knew then that Adam was an arrogant man who had lied and lied.”
Ms Flounders initially agreed to support Johnson before the trial and was photographed in February 2016 with the footballer as they left Bradford Crown Court.
The 29-year-old said she received abuse on social media because of the picture.
“People said I must be getting paid by Adam, that I was a gold digger and a stupid bitch,” she said.
Ms Flounders gave evidence during the trial, telling the court that the couple had broken up and Johnson had admitted to cheating on her with “quite a few” women.
She said the former couple would remain friends following his release for the sake of their four-year-old daughter, but insisted she would not take him back.
“You can forgive but I will never forget what he has done,” she said.
“I would struggle to trust any man now.”
Agencies contributed to this report