the problem with your potrait is that you dont understand the 3d structure of the head. i did some very quick redraw of the 3d structure of the head to get you the idea how the 3d structure of the head need to be.. and because of that you're struggling with the shading of the head because when shading you need to understand the form of the object you are shading so thats something i feel you need to work on.. hope that really help you with the potrait drawing











