Perfection is the enemy of mastery. Just learn one thing at a time. Start with proportions. Move on to light and dark. You should be able to create a semi-recognisable portrait with next to no details. Only when you've got to that stage do you move into any detailing at all.

You'll learn a lot more doing lots of fast exercises, than trying to create a master piece. Finish sooner, and you'll accelerate faster