The 'Something New' singer was petrified about returning to the public eye for the girl band's final 'Ten: The Hits Tour' earlier this year after completing rehab in October 2011 to fight depression and alcohol and sleeping-pill addictions and almost crumbled under the pressure as the group were about to make their big reunion announcement in front of the UK's media.
Writing in her new autobiography, 'Whole Lotta History', Sarah's bandmate, Kimberley Walsh, explained: ''She'd had a couple of drinks to calm her nerves, which didn't help matters. Sarah can be unpredictable when she's nervous and we were all very anxious about her state of mind.
''She really was in a bad way. ...Of all of us, she had the most to prove. Sarah wanted to let everyone know that she could do it, and that she was back on her feet again.
''The tears were starting again and I really felt for her but this wasn't the time for sympathy or sentimentality ... Being hard on her was the only way. 'You can do this,' I said, firmly. 'You have to. You've done it a thousand times, and you can do it now.' Sarah nodded and pulled herself together just in time.''
Kimberley,31, was worried about Sarah rejoining the band and feared the stress of performing tricky dance routines live would affect her recovery.
The singer said: ''[Sarah] was very nervous about getting back in the saddle as a live performer after three years, and I could see she was struggling.
''Sarah had never been particularly confident about the choreography side of things, and now she was even more out of practice. I felt for her. Unfortunately, Girls Aloud had become a very visual group and choreography was part of the package.
''I even wondered whether a tour was the right thing for her to be doing - it became a moral dilemma. I wanted the tour to be amazing, but not at the expense of Sarah's wellbeing.''
The novel's author saw a cut of the film and loved each of the changes the movie's director had made.