I agree with your own assumption, Adam. In my opinion what you currently need, is an understanding of the general proportions and planes of the head.
At this stage, I would not concern myself with shading yet.
I‘d recommend you redirect your focus from doing pretty pictures to attempting to understand the subject matter youre trying to draw.
What helped me the most in understanding the structure of the head were some videos by Angel Ganev on youtube:
Once you‘ve understood the structure of the head, shading correctly will be a lot easier
Also: its obligatory to do these exercises WITH reference, by doing that, you‘ll develop a good sense for the general proportions of the face and you‘ll be able to spot individual differences with different models and refs.
Good luck, I hope that helps