Merrygold, whose band JLS were runners-up on last year's series, started dating Natalia Rush shortly after they met on a TV set in October 2007. The pair stayed together throughout the X Factor live shows in 2008 but finally hit the rocks last month.
Rush, 19, has now revealed that she became infuriated when she realised that the singer was in regular contact with his former partner Lytton, who is best known for playing Ruby Allen in EastEnders.
"All through our relationship there were three people - Aston, Louisa and I," Rush told the News Of The World. "We had a blazing row which led to me dumping him. I got back with him after he promised he wouldn't hide anything again. I could deal with Louisa being his ex but she was always there or ringing him up.
"I repeatedly told him, 'She is your weakness'. He hated me for that but he knew deep down I was right."
The teenager added that she had another row with Merrygold after discovering that he had broken his promise and continued to contact Lytton. They finally parted ways in January when the boyband star decided to end their relationship.
"I feel so used. I supported him when he was a nobody and now he has tossed me out of his life," Rush explained. "I don't want to know him any more."
JLS recently signed a record contract with the Epic label.