If you can't do the straight runs on one ski, time to start looking at the shims again.
Take a roll of duct tape with you and start shimming one side of your boots a little at a time. (hopefully you remember what side the shims were on) 2 strips of duct tape = about .5 degrees
Once you can comfortably ski straight runs, then try the one legged turns again.
Straight runs a-ok , strangely the shim combo that makes this the easiest and feels most stable makes turning to the weak side harder and vice versa. ie with shims under the insides of my feet my ankle is very stable and I get on
edge very efffectively but, cannot make the move in question. Conversely I can sometimes turn to the weakside when the shims are on the outside but my presentation doing a straight run is that the ankle collapses to the inside. (see below)
Actually, go ahead and do both sides. If it feels worse going one way, you'll know to go the other way. You know that you can ski around a lot of messed up stuff. I think you need to start with footbeds though. The feeling of "collapsing" when standing on one foot is one I know well, and correcting it started with footbeds. FWIW - it took me 4 sets of footbeds to get there. Once I felt ones that were correct, it was pretty obvious what had been missing. I also found that the abducted stance boots (which you seem to be wearing) made standing on the LTE much harder because rotating the femur outwards moves the ankle inwards and biases you toward the BTE (that's why the boot is built that way).
How'd that go?
One of the balance board drills that I do is standing on one foot. Starting with the board tilted to one side, I roll to flat, pause, then roll to the other side. I had to start doing this drill with my hand touching the wall in front of me for assistance and gradually worked my way down to one finger then no touching.
Can you do straight runs on one ski?
Do you have a front view picture of yourself in shorts standing in bare feet?
What model boots do you have?
ok not as well as I'd like but better than i expected. I tried to watch my reflection to assure the change came from the hips down. it reminded me of the sensation of when the com crosses the skis moving into a new turn. I suppose that is what this task is all about.
My personal experience and observations have convinced me that skiing on one ski on the LTE is anything but natural. The movements that are normally used to turn in that direction on two skis get subconciously thrown out the window; tipping of the ski usually comes from tipping the entire body rather than tipping ski using ankle and leg movements alone.
When you are on one ski, you have a very small BOS. Since you start, presumably, from a position balanced over the BOS, when you tip your skis by tipping your entire body and don't get sufficient turning response from the ski (which is typical), you will fall to the inside. Acheiving effective edge angle on a single ski is difficult if you are falling to the inside.
Exageration is often suggested to get athletes to end up in the correct, not exagerated, position. People will often think they are moving feet when they are moving inches, so you ask them to move miles to get them to actually move a foot.
You can exagerate your (correct, two ski skiing) upper body movements to counteract the natural tendency to simply tip your body to the inside in order to simply sustain the movements you would acheive with two ski skiing. This way you will keep your COM over your BOS which results in the ability to control the ski through edge angle management.
It will feel like good skiing, but it won't feel natural at first. My drill to conciously sense your upper body movements then to start one ski skiing without stopping, will help you maintain the upper body movements needed to maintain COM over BOS as you go from two skis to one.
Also important to note is that maintaining some flexion and counter fascilitates the tipping of the ski through ankle movements. These fine movements are essential for maintaining your balance over the small BOS. If you are straight legged (or nearly so) and developing edge angles from tipping the entire body, you have less chance of correcting your turn shape in order to maintain balance as your entire body has to move to change edge angle rather than just your ankles.
Off snow, simply try balancing on one leg. You will probably find your ankle will be working furiously to maintain balance. When you can balance without your ankle having to work so much you will likely find it easier to ski on one ski.
Sorry, what does BOS stand for.
Epic and MR are correct: one-skiing done correctly should be "just good skiing." There are many people who lost a leg and still ski at a very high level, though rare to see in the wild. Before Weems Westfeldt got his new artificial knee, he skied all terrain on one ski extremely well, the other one was just a pretender. But for many of us, one-skiing is a L3 task we do simply to pass, on the level of a dog and pony show trick.
Not! Actually I can't think of a better task to teach finesse and emphasize transitional movements. First, pick appropriate terrain--summer roads are great for this, especially if they aren't major access routes. Second, visualize what you want the ski to do: tip to the right edge, pressure the ski to make it turn, flatten, tip to the left edge, pressure the ski to make it turn, repeat. Rehearse the movements with the same side hand as the foot you intend to stand on.
From your photos, in what looks to be rather difficult snow conditions, it appears that you could be a genital retreater, which is to say you tend to "park" your hips back under your body either habitually or as a security measure. As you say, one-skiing is difficult for you, so perhaps you are skiing a bit defensively when you do this task? Whatever the psychological motivation, one can't one-ski without allowing the hips to extend forward and across to release the uphill edge and the pelvis to tip from side to side to match the slope angle and maintain alignment on the uphill edge.
So, my advice is to allow your hips to extend forward and diagonally to assist the edge change and to tip your pelvis to assist alignment on the edge (i.e., the right side of the pelvis is raised to the same angle as the slope when you are riding the right edge and vice versa). I suspect this is what's missing from your present approach.