I literally had the same problem as you for almost a full season. The one tip I tried that really helped me develop my stance was thinking about leaning on my forward foot. Whenever I did that, I found instead of having more weight on my front or back foot I'd land on the rail much more centred just from thinking about putting more weight at the front.
Another thing I'd try is focus on the boxes at first, than you dont have to be as concerned about balance on the actual feature, but just balancing your body. Once you get the feeling of sliding sideways on a box and your proper stance everything else comes much easier.