You need to sit more like doing a squat it will help with the kicking of your foot. Also you seem to be sitting back on your board and that will lead to a loose feeling when you get to speed. Just have someone film you or watch you and they can tell and show you want you need to change. There is also nothing wrong with getting a lesson and telling them I just want to work on carving with my edges.
Edit: You can also try 18/15 that might help you also