The Eurovision star was recruited as a new member in September following the departure of ousted singer Keisha Buchanan.
Following her appointment, Ewen immediately flew out to Los Angeles to film a video for the band's single 'About A Girl', before hitting the recording studio to replace Buchanan's vocals on forthcoming album Sweet 7.
Reflecting on the experience, Ewen told The Scotsman: "I felt really restricted, actually. You've got the part already designated to you, a part that was never written with me in mind so it's not necessarily suited to my voice and what I can do.
"So I felt like I couldn't really demonstrate what I was capable of. And on top of that you have criticism of course, which you get all the time - people saying it's not as good as the original. But of course it's never going to be as good as the original because it wasn't written for me. All I can do is put my spin on it."
Amelle Berrabah, who joined the group in 2005 as a replacement for Mutya Buena, agreed that it can be difficult to be "constantly compared to someone else", but added that Ewen has done "an amazing job".
Sweet 7 is released on March 15.