This is almost certainly a technique issue. The fact that you are saying you really need to 'throw your back foot around' screams technique problems. It's hard to pinpoint the exact problem without seeing some footage or a better description of what's happening, but my first thought is get more weight onto your front foot. You want your body perpendicular to the angle of the slope. i.e. equal weight on both feet, even when riding steeper slopes. You should be initiating your turns with your front foot/leg/hip, never 'swinging' your back foot around.
Is there anyway you can upload some video of yourself riding? Or maybe a detailed description of what is happening? I can keep throwing suggestions at you.
Also maybe have a read of this article:
http://www.boardworld.com.au/snow/content/category/basic-turns/
You can also have a read of 'Steering':
http://www.boardworld.com.au/snow/content/category/steering/