As everybody else stated, your weight isn't balanced which caused you to slide out.
To fix it quickly you'll need a hikeable box or rail and shit ton of weed/bengay/icyhot/Vicodin(use Vicodin AFTER...!)
Basically hike the feature and boardslide it constantly. Start by 50'ing on and then moving your board into a boardslide, then when you feel comfortable pop into a boardslide. You'll fall a lot at first but eventually you'll get a feel for it and soon enough you'll be able to boardslide anything. Also if you widen your stance a little you'll gain some stability.