Hey there! First of all, I think your struggle might be cause by focusing on multiple things, like construction, shading and proportions, at once. Doing studies like this, I'd suggest not caring about the shading at all when starting of. It's enough to keep the local color in mind and you can always focus on values and volume, once your general structure of the face is mapped out more believably. Instead, try to focus only on the overall features you draw and carefully observe the relations between them with your reference. In the overpaint I mainly tried to achieve a more believable face by adding more information. The most significant change being the eyebrows and the neck area, because the darkness of the beard made those areas look underdeveloped. I hope this helps and keep the studies up. You'll see progress quickly doing more of these!
