If you've spent much time riding motorcycles its even harder to learn how to corner a standup than starting from scratch. I find it really hard after riding my r1. on a bike you lean in but steer out, on the ski you have to keep the bars turned in to the turn or it will just straighten up and you fall in the water feeling confused. Theres no gyroscopic precession to use in the corner on a ski, its all power and where the jet is pointing.
You're preaching to the choir on that one. I think initially it was way harder than starting from scratch because I tried to carry over all of the MX habits. Even with counter steering aside, it is still entirely different.
I started on a 550SX and I was leaning too much to the inside early on, and tipping the pump out of the water, and then it falls over at the loss of power.
I'm by no means good at fast turns yet, but I am starting to get the hang of them! Moving to an SJ was such a difference, it rides on rails like nothing else. I think it may be easier than the 550, but maybe that's because I already started to get the feel. The skis are both so different in so many ways.
The first time I rode the SJ I cranked it hard under power and it threw me right off the outside like a bucking bronco. That's how I learned how well it hooks up.
I agree with the other comments, maybe trying to ride super slow in a straight line, then turning might help get the feel for leaning and saving it from tipping over. Practice, practice, practice!