The Crazy Stupid Love singer was axed from the TV programme just weeks into her high profile job - but she insists she "wasn't well in the head" at the time.
Cole 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 desensitised, 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 star, who raised eyebrows when she appeared with big hair and dressed in purple trousers and an orange frilly top on her first day's filming for the US show, sent Cowell a text after she was dropped.
"F**k you, F**k Fox, F**k the orange and purple outfit. F**k the big hair. F**k the UK X Factor. F**k you all. I hate you," she wrote.
Cole admitted she felt as though she was having a nervous breakdown last year.
"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?'," she said.
"I didn't know what I thought or felt about anything, couldn't make decisions - I didn't even know what to eat or drink.
Something was seriously wrong: it felt like a nervous breakdown. I had to stop, get out of my life I'd been living and find myself again. The consequences if I didn't were just too frightening."
The former Girls Aloud star, who divorced her husband Ashley Cole in 2010 and is now dating French restaurateur Jean-Bernard Fernandez-Versini, said that today she is a stronger woman.
"I thought I had every box ticked, career and personal. I was the first to get married. I was the one everyone thought would be the first to have a baby and then the rug got pulled out from under me. I've learned that there is no such thing as a plan. You can't make them," she said.
The singer added: "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.
"It actually taught me everything I'm going to need for the rest of my life. I love feeling that I'm strong because I have good women at my side. And I don't give a f**k what people think any more."
Cheryl Cole returns the X Factor, which will air on UTV in August.