The star, who married the Westlife singer in 2002, said that she was in despair when he decided to split from her two years afterwards.
She told the News of the World: "That killed it for me. I loved Brian and I trusted him. I remember thinking: 'But he's the fat one from Westlife. Why would he do this to me?'"
Katona had two children, 8-year-old Molly and 7-year-old Lilly-Sue with McFadden, before he decided to leave her for Australian singer Delta Goodrem.
The ex-Atomic Kitten singer continued: "I thought we could work through it, but after Lilly was born, Brian told me he didn't love me. I didn't know it then but he'd met Delta.
"I didn't fight. I saw the way he looked at Delta. He had never looked at me that way - I knew he had gone."
She added: "I remember thanking God I'd dressed up because this vision with a perfect figure, white shiny teeth, and a beautiful complexion was next to him. She was about a foot taller than me and I felt like a piece of rubbish.
"I was so nervous all I said was, 'Hiya love, are you all right?' This to the woman who had taken my husband."
