Hello there! You have a good foundation here! I just tried to add a bit more colour vibrancy to the skin, such as reddish tones on the cheeks and a subtle touch on the nose, as well as defining the shapes of the nose, cheekbones, etc., a bit better. If you're having trouble defining the shading of the portrait, I recommend checking out the Asaro Head. It will help you understand the planes of the head and how they're shaded according to the direction of the light. :) Also, try to incorporate both hard and soft edges. Use hard edges to define changes in planes or cast shadows. Hope this helps!