Face planes above is a great piece of advice! You can also use a program like Daz as reference on a screen next to you while you work. If the work above is for training then I don't think concentrating on just a head for such a long time is a good idea. You should be making a lot of figure sketches to get comfortable with…
While the face normals (the one from a center of each triangle ) looks perfectly up those normal are derivative as a median of vertex normals in triangles corners. The bottom left corner for example , clearly not 90 degree up . it's what creates shading gradients. Face weighting, or direct normals editing could fix it .…