I would have someone take a look or a quick video of your riding, dont do anything different than you would normally do when you get video'd. it's possible you may be trying to 'face' the fall line with your body instead of your head.
Due to an ever pressing issue where my body feels the need to slowly kill itself, my joints are all screwed up, including my neck. If I turn the way I should, it hurts and thus, I have a bit of a body turn which hindered my ability to ride properly. I knew this as when I would go fast and get abit down in my position, this would force my body more forward and I found I had more control then.
I finally put two and two together and that was some of my issue. This also caused my left knee (reg rider) to be fatigued more after 6 hours of riding with a few 20 minute breaks and a few on the hill breaks , on top of the lift breaks. I figured well my arthritis couldnt be that bad already. Sure enough I had too much twist on my knee all day.
Try to lean back just a tad, turn your head a bit more left and this 'should' set your body in line with the board better.
A lesson from a good instructor with your concerns up front should allow them to point out your stance flaws; however, I havent seen many snowboard instructors out east that know what they are talking about so, thats another topic depending on where you are. I believe there is a certification to look for but dont remember what it is. Hitup snowolf on these boards, he'll tell ya