For learning the structure of the head shading helps a lot, because it makes the result more 3D and shows if your underlying lineart structure was good enough to shade it right
That said I think you're like me and use the reference to build up the shading instead of your lineart.
I guessed that because of the cheekbone line in your latest drawing. In your lineart it's very close to the eye (I think the endpoint of that quarter circle is too high maybe?), but in the shaded version the shadow beneath the cheekbone is placed at the right spot again
Maybe it could be a good practice to use the reference for the lineart and construction and to detect the direction of the light source, but do the shading yourself? And then check back with the reference.
I don't do that myself yet, but I think I should try that as well