Keep falling forwards or veering off to the side on one foot glide.

I am trying to glide backwards on a flat, then later on an edge. I can glide fine with my leg out, but my teacher wants me to keep it in, with the ankles together.Every time i do this my back twists and i bend forwards. Or, if my back is up straight I instinctively bring my leg up and in towards my core. I just curl up.Or i twist. I cant hold myself straight. My shoulders go to the side and i bend with them and fall over. When i tried to hold them still my hips, or foot, would twist. When a friend tried to hold me straight and help me with my glide, my foot twisted again and i ended up doing some kinda 3 turn.Idk what I can do to fix this. I can glide fine on one foot if i can have one leg behind me, but not with them together like this, with the ankles together.