Demi Lovato had self-esteem issues since she was "three-years-old in a diaper".

The 20-year-old singer and US The X Factor judge spent three months in rehab in 2010 for bulimia and self-harm.

Demi now has a positive self-image, but finding self-confidence has been a lifelong struggle for her.

"I remember being three-years-old in a diaper and rubbing my hand over my stomach and I remember thinking in my head, 'I wonder if one day this will ever be flat?' " she revealed in an interview with Katie Couric on talk show Katie that airs Monday.

Demi has been named ambassador of Secret deodorant's Mean Stinks anti-bullying campaign.

The star recalls a time when she was absolutely tormented by her cruel peers.

"[The taunting from my schoolmates] played more of a toll on me than if I was physically abused in school," Demi said.

"I've always said I wished that they had just hit me in the face and gotten it over with because what they said to me sticks to me to this day and it affected me, [turned] me into the person I am today. I was bullied and they called me fat and they called me horrible things."