The pair famously fell out when Cowell sacked the former Girls Aloud star from the US X Factor panel after only a few days on the job.Cowell has told Radio Times magazine the singer "had become unrecognisable" and "had lost her confidence" following her divorce from footballer Ashley Cole.He said: "My falling-out with Cheryl came soon after her divorce and was a nightmare. We were all wound up."There was so much expectation on the show in America and I definitely wasn't myself. I was uptight."Fernandez-Versini's appearance during the first day of auditions for the US show made headlines when she arrived on the red carpet with big hair, purple trousers and an orange frilly top.When I saw (Cheryl) I didn't recognise her. Literally. The hair and outfit were crazy. Just totally crazy.Simon CowellCowell added: "I asked her manager, 'What's she wearing, what's happened to her hair?' He was defensive about it. I haven't a clue why she turned up like that."He continued: "She'd put on a bit of weight, but not loads. That wasn't the issue. The only way I can describe it is that it wasn't Cheryl sitting in that seat. She'd lost her confidence."Cowell admitted of the US X Factor, which ended after three series and falling ratings: "In hindsight I don't think that first day worked anyway. There was something not quite right about that show in America."He said that Fernandez-Versini - who is back as a judge on the UK X Factor this weekend - "turned on" him when he mentioned her outfit and told her that her first few days on the US show had not gone well.She later told him that he had "betrayed" her."If I could go back in time I would have kept her on the show. It was stupid doing what I did. I didn't sack her personally. That was the second-biggest mistake I made. When you're under pressure you make stupid mistakes," he said.Fernandez-Versini has previously revealed that she sent Cowell a "very abusive" text message after being sacked from the show.Cowell told the magazine: "The worst it got between us was when she sent me a text message.It was like when you get a really horrible letter from an ex-girlfriend.Simon Cowell"The general tone was something along the lines of "you've hurt me, you've let me down, I can never forgive you". I shrank."Fernandez-Versini said of her sacking: "In hindsight he was right because I wasn't myself. He wasn't himself either. He was under a lot of strain because it was a new venture."But he handled it wrongly - he got somebody else to sack me. I didn't want to talk to him at the time. I didn't want to talk to anybody. The sacking came on top of everything else, it was just the cherry on the cake."She said that the music mogul - who got someone else to sack her - found confrontation difficult because he is a "good person", adding: "I've warned him never to let me down again. Just let him try."But in a dig at her boss, she added: "He's more ruthless than me but I'm stronger in my opinions; I believe what I'm saying."The X Factor returns to UTV on Saturday and Sunday from 8pm.