The star, 31, was left humiliated when she was axed from the show just weeks into the job and before the series aired.
She told Elle UK: “I can talk about this because I’m OK about it now. But I went through hell and it literally drove me mad.
“I became so de-sensitised, you could say terrible things to me and I wouldn’t even think about it. I’d read awful stuff about myself and not blink. I shut down because I didn’t know what else to do.”
She told the magazine: “Now I can say that Simon was right to get rid of me.
“I wasn’t well in the head when that was going on, so it wasn’t going to work. But he wasn’t right not to tell me to my face.”
The usually chic star, who raised eyebrows when she appeared with big hair and dressed in purple trousers and an orange top on her first day’s filming for the show, sent Cowell a text after she was axed.
“Fuck you, Fuck Fox, Fuck the orange and purple outfit. Fuck the big hair. Fuck the UK X Factor. Fuck you all. I hate you,” she wrote.
Cole, who is now back as a judge on the UK X Factor, revealed that she felt as if she was having a nervous breakdown.
“It only hit me at the beginning of 2013 that I’d completely lost myself. I’d look in a mirror and think: ’Who the hell is she?’”
The former Girls Aloud star, who divorced Ashley Cole in 2010 and is now dating French restaurateur Jean-Bernard Fernandez- Versini, said she is now a stronger woman.
“I’m just enjoying finding out who I am. That’s good enough right now... I know I’m really lucky because none of what I’ve been through broke me.”