Demi Lovato believes it will be difficult to replace Britney Spears on the US 'X Factor'.Britney, 31, revealed last month that she won't return for the next season of the reality TV show but Demi is worried about finding someone who can match the 'Toxic' singer.She told E! News: "I mean, who can replace Britney, let's be honest."Â Britney announced her decision to quit her role as a mentor on the Fox show in order to focus on her music career and admitted watching the contestants perform every week helped her to make the decision to leave.She explained: "I've made the very difficult decision not to return for another season. I had an incredible time doing the show and I love the other judges and I am so proud of my teens but it's time for me to get back in the studio. Watching them all do their thing up on that stage every week made me miss performing so much! I can't wait to get back out there and do what I love most."Demi is expected to remain on the panel with Simon Cowell, although fellow judge L.A. Reid has also quit.