Art of Chandrasana

Lately in class we've been practicing ardha chandrasana a lot.. It looks so simple... but boy is it hard. As soon as you think you have your balance, there it goes. Also, it seems easier to do this pose in an empty room alone... I can do it, right now, no block not even a wobble but get me into the studio and I look like a fish out of water. Why is that? I haven't the slightest clue. If a tree falls in the forest...