She said: "Well it was overwhelming because never in my wildest imagination did I think I’d sell that many records.
"But to sell all of those in such a short space of time when I still didn’t really feel like a professional singer, I was just learning my trade. I was pretty speechless which is rare. It took a while to sink in."
For Susan, time has flown by since she appeared on 'Britain's Got Talent' and she admits it feels like a different lifetime.
She added: "Thinking back to 2009 it feels so very long ago, a totally different era, but yet at the same time it feels like yesterday I stepped on to the 'Britain's Got Talent' stage.
"That year was about learning and adjusting and ‘baby steps'. I must have said that phrase hundreds of times over, I’m surprised people didn’t think I got my shoes in Mothercare!"
Susan, 50, has also revealed her third album 'Someone to Watch Over Me' has been recorded as a response to all the letters she has received from fans around the globe.
She said: "The album itself was like a journey of self-discovery. It takes you on the emotional roller-coaster of life.
"I only wish I had the time to respond to each and every letter. One thing that struck me was that it was folk of all ages who wrote, from 20 to 90 years of age and each letter was an inspiring insight into the lives of these people.
"The music on this album reflects the sadness, happiness and immense bravery of these people."
'Someone to Watch Over Me' sees Susan sing tracks including Depeche Mode's 'Enjoy the Silence', The Righteous Brothers' 'Unchained Melody' and Tears for Fears' 'Mad World' and is released on November 7.