it is happening because you're not shifting your weight to your front foot. You must have the weight of your body on your front foot first (just like you're standing on it), then move your back foot and shift your weight on it. repeat. try standing not on the sides of the board but more in the middle so you have balance. good luck!

First try to drive as far as you can with keeping your balance on the front foot at back of your board. Than start to learn on.(believe me I had the same problem)

I'm not a pro but start by practicing just doing it whilst still up and back down and than try to do it whilst moving Hopefully this has helped it's what I did so ye

If you can do it stationary on concrete then u have enough balance to be able to slowly build up speed