The Saturdays star told the News of the World that she "was having a bit of a crisis" which prompted the separation in March.
"We'd been living together for nearly two years and it felt too much too young. I thought I wanted to start partying and being like girls my age," she confessed.
Sandford admitted that she quickly regretted the decision.
"It all went horribly wrong," she said. "Every day there was a story about me and a new guy in the papers. I'd never had to face that before and I couldn't bear it."
The 21-year-old also discussed reports which linked her to Cheryl Cole's ex-husband Ashley Cole.
On the claims, she said: "As if! I've never even met him. I'd get upset and I'd call Dougie and tell him none of the stories were true. We were still talking and texting all the time.
"I got more and more cut up about what I'd done."
The couple eventually reunited and are now thought to be buying a house together.
"I never appreciated what I had until I threw it away," she said. "I've been lucky enough to have a second chance. I'm still too young to get married but he's the one I'd like to have kids with some day.
"He's kind, funny, loyal, protective. Now I know there's no-one else like him. I'm not losing him again."