Replying to Help!

before you read this please keep in mind that this is my first year freeskiing and i need a lot of help so please dont flame me!!!!!

i've been trying to pull a 180 on a tabletop but i can never quite get it. i get about halfway thru the rotation and my legs fly out from under me (probably becuz the tabletop is no longer there) and i end up falling on my side onto the tabletop. can anyone explain to me what i'm doing wrong? am i going to slow during the rotation? should i pop before i get onto the tabletop?