The 'Say You Won't Let Go' singer was famously dropped from his Syco record label after a very public fall from grace and now back with the label, James admits he was a ''t****r'' back then.
He said: ''I did apologise to Simon. I was lucky enough to be given a platform [with 'The X Factor'] and it's done amazing things for me and then I went and handled that irresponsibly. I feel I owe a lot to the show and I apologised for being a t****r. He's much more down to earth than I imagined he'd be.''
And the 29-year-old singer has ''a lot of respect'' for Simon for being so ''gracious'' and humble.
He added: ''I was just on the final of America's Got Talent and it's the biggest show in America, and to me it was a massive moment - this is one of the highlights of my career.
''I'm singing on telly in America to [millions] of people, and we were in Simon's dressing room after the show and he was like, 'Look at us, we're just a bunch of British people here in America doing really well.'
''I thought that was so gracious and even he, after all his success, still values those moments and that's why I have a lot of respect for him.''
James admits he ''took himself too seriously'' in the beginning and says fame happened ''too quickly'' for him.
Speaking on the upcoming episode of 'The Jonathan Ross Show', he shared: ''Things just happened too quickly for me ... I went from living in a little bedsit and not having many expectations to having a lot of judgment overnight. I just totally lost it ... I just took myself too seriously I think.''
'The Jonathan Ross Show' airs on Saturday evening (30.09.17) on ITV.
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