The only reason I started making 1 handed turns was because my ski coach told me to. I had been told that it isn't necessary until '28 off and frankly, I was a tad scared to try it and get the rope yanked out of my hand.
I'm not knowledgeable enough to say for sure what you should do, but my first guess would be you could try squaring your shoulders to the boat. If you try a one-handed turn in the position you're in now, I could see how you could easily fall since your left shoulder is far lower than your right, causing you to be off balance. But if both shoulders stay equal height and facing the boat while your hips and legs rotate for each turn, you stay balanced and expel minimum effort. Here's what I was first told regarding the turn:
"Let go of the handle when traveling outwards, letting it dangle in your hand. Move your now free hand to rest on your hip. turn by "skiing back to the handle". Don't move your free hand off that hip, move your ski to meet it."
My coach tells me "ski back to the handle" all the time because I always forget it, but it works so well. Instead of fighting the boat and pulling that handle back to your free hand and then forcing the ski to turn, you just leave the handle where it is and move your ski to catch up with it.
I hope that helps/makes sense. Like I said I'm probably not good enough to warrant giving advice yet.