The singer and actress, who is dating Justin Bieber, explained that she allowed an ex-boyfriend to influence her too much, but would never make the same mistake again.
Gomes told Top of the Pops: "I wasn't myself for a year. I think that was really damaging. I felt like it was me who was always trying to be perfect for him.
"Everyone around me hated him but I didn't listen. I think I had to go through it because now I'm the strongest I've been. I dare someone to try and change me again."
Gomez admitted that she lost herself in the relationship because she was constantly trying to please her partner.
"I always dressed nice. I don't think he ever saw me in sweatpants - and I love sweatpants," she commented. "I'd always be well-mannered and go to his favourite restaurants or whatever.
"There was a point where I lost myself completely. I didn't smile as much, I didn't laugh as much. I wasn't a goofball. I didn't get up and dance like I usually do.
"And when that one ended, I didn't know where I was. But I got it back and was really happy. My family were happy to have me back."
Gomez recently said that she sees London as a "second home".
Watch Selena Gomez and The Scene performing a Britney Spears medley below: