He's an observation. Take it for what it is from an intermediate rider that only ever taught one kid to ride. But I watch people ride and had similar problems as you.
Typical thing that happens to many riders when changing onto their heel edge is that they force the direction change of the board too early. When you change over to your heel edge you use your body to point the board to the left side of the piste. You can't do this when you're in the back seat, so that's not the problem. you're pivoting around the front foot. Then after you do this direction change you don't put enough pressure on your edge with the back foot, so you keep sliding out.
Watch a carving tutorial or just a video like the Korua ones. When you tilt your board to switch your edge, the board has to keep traveling in the same direction as you were going in. Then once your edge bite the board will start to turn over to the other side. But this means that in the beginning of your heel side turn you will still be going towards the right side of the piste.
If you're going to be sliding your turns because it's too steep, do a test where you press your back foot heel down and lift your toes on the same foot. Just to see if you can get the edge to bite.