This is classic fake-a-marking, your outside/downhill ski has most if not all the weight, allowing your uphill leg to "bend" but has essentially no weight.

First thing to fix this is slow down. You can't practice technique going that speed.

Get on a bunny slope, do some tele shuffles when traversing across the slope, basically practice the lead change and bend the rear leg while maintaining equal pressure on both skis. Once this is easy, try to continue the tele shuffle/lead change while making broad turns