You have a really nice style and a great vision, and I think your self-diagnosis is accurate. I can see there's a lot of hesitance and uncertainty in your shadow forms. There is a very strong shadow under the chin in the left drawing, but the way the rest of the face is shaded, while not wrong by any means, doesn't follow a rule you've unintentionally set by adding that one deep shadow there. That rule being, if there is a strong, deep shadow under the chin, there must be a bright light coming from somewhere above to create it. What I did here was add a multiply layer to add the shading on the left drawing. If you're worried about ruining your piece, either make a copy of the file or experiment on a new layer. Digital allows us a lot of freedom, so make the best of it! For future study I suggest playing around with some 3d head lighting reference tools, they can be found easily through google search and usually run in your browser. Move the light sources around and see how the shadows on the head respond. Keep at it!
maybe pick one clear light direction and push the planes that turn away a little more, especially cheek and neck, before worrying about every shadow. the mid values can stay readable while the form gets rounder.
thank you very much!
idk if every shadow needs to match the chin depth. maybe lock one bright plane on the forehead and cheek first so the light direction is obvious, then keep the side of the face softer so it doesnt go muddy
idk if the whole face needs that chin level dark, maybe deepen just the neck and under the brow so the cheek stays a mid plane and still feels round
yeah the chin shadow idea helps, maybe keep the light a little higher and let the cheek turn through a softer midtone first, then push the neck shadow so the volume reads without muddying the face