I don't know exactly whatyour head drawing practise looks like, but I do agree with Lesley, that slowing things down, will most likely help you a lot.
I have the feeling that you are trying to do everything at once (basic form of the head, facial features, etc.).
It might help to break down this big skill of constructing a human head, down into smaller managable tasks, so you can focus on one thing at a time and do it well.
For instance, what does drawing the basic form of the head entail?
- being able to draw a circle (great, focus on that and only that until you consider it good enough to move on)
- determining a side plane for this sphere
- drawing the center line of the sphere
- attaching the jaw
- drawing in the nose, hair and eyebrow line
- getting the proportions of the head right
See there is a lot of stuff you could practise
I can only speak from my own experience and I do not practise nowhere near as long as you do, but I focused on these skills mentioned above drawing 2 heads max within 60-90 mins every day, continually comparing it with a reference image and even measuring out the proportions. Its the reflecting, the thinking about what you've done and the comparing with reference that makes you LEARN things you're practising.
Volume is important, but making the same mistake over and over again, because you dont think about what youre doing is a waste of your time...
Work hard, but more importantly work smart, mate!
You've got what it takes!