Olympic gymnast Shawn Johnson, who stole the hearts of millions at the 2008 Beijing Olympics, retired from the sport on Sunday.

Johnson, a four-time Olympic medalist, retired after being unable to overcome a debilitating knee injury suffered in a skiing accident in 2010. Johnson suffered a torn ACL and MCL in the accident and was never able to recover, despite an enthusiastic comeback attempt.

This is a really hard moment, Johnson told USA Today. I still have the heart, drive and desire to compete. Unfortunately my road has been cut a little short.

It was actually the skiing accident that propelled Johnson, 20, to thrust herself back into competitive gymnastics. After the Beijing Olympics, Johnson returned to the United States as a celebrity and became busy with media appearances, endorsements, and more. She won ABC's Dancing With the Stars in 2009 and distanced herself from serious competition before the accident.

She realized that she still wanted to compete in the Olympics, but her knee just couldn't hold up.

It's been a constant fight. It came down to there wasn't enough time left to heal, Johnson said. I couldn't push myself any further. My knee physically wouldn't let me do anymore.

Although it was quite a difficult decision for Johnson to retire just weeks ahead of the 2012 London Olympics, Johnson appears upbeat. She plans to attend the games and root on her former teammates, as well as her training partner Gabby Douglas, who is expected to make the team.

She also plans to attend college in 2013, but is still deciding between Vanderbilt and Stanford. Beyond that she doesn't know exactly what she'll do without gymnastics in her life, but she's excited about the possibilities.

It's hard to accept that the one thing you had your entire life is no longer there, she said. I'm scared to death to know what the future holds. It's a road I never thought about. I'm excited as well, ready to start the new journey.