You could take a look at the models by Andrew Cawrse, good but expensive.

There is nothing wrong about using whatever tool to enhance art. Just be honest about it.

Perspective looks wonky, how about leaving your construction lines for all of us to check?

In my opinion, drawabox is great for developing eye-hand coordination: I really wish I had known about this in animation school, where I realised my motor skills sucked big time, while my teachers kept repeating that I should draw more. However, after the first fistful of exercises, drawabox is less useful. The value of Drawabox is not in drawing boxes.

As a beginner, use any medium you're comfortable with. So, if you feel Krita gets in the way, leave it for now. In general, there is nothing fundamentally wrong with digital, just don't overly rely on undo. Try to get it right first time.

I don't think gouache is an alternative, as it is opaque. Acrylic springs to mind, possibly using an medium to extend its open time. Acrylics dry fast in comparison with oils, though: it needs some practice to get used to it.

Your sketchbook is your sandbox where you can play, practice, fail without anyone noticing. It is not a portfolio of your best pieces.

Search for the course 'Portrait Drawing Fundamentals'

Study formal perspective long enough for it to become intuitive.

Measuring is fine, but be aware you're not a scanner: find peace in making mistakes, and be aware that you don't have to be 100% accurate to be good.

The rule mentioned by Robertson and others is a rule of thumb. It is not mathematically correct. However, it is the only practical guideline known to mankind, so let's stick to it...

The perfect is the enemy of the good!

Art can be about anything, and Mathematics is just another topic.

I went to art school at age 42. In my experience, you will learn slower, in particular when it comes to motor skills, like eye-hand coordination and line quality; also, you may have less energy than the kids around you. However, you will probably learn more efficiently, due to experience with planning and self knowledge, and you are likely to be better motivated than your younger peers. You may run into preconceptions from employers not hiring you: age discrimination does exist, unfortunately...

Sometimes an honest opinion is just an honest opinion...

In my experience, the overhand grip allows you to separate the location of your medium, using shoulder and elbow, from its orientation/character, using wrist and fingers. The latter is particularly relevant when using a softer medium, like charcoal or Conté, on an easel. In addition, I see it as a teaching tool, forcing students to separate the two mechanics. Once you get it, anything goes. I know people drawing really well from the dreaded monkey paw, which I do not recommend at all for beginners. And, of course, using a stylus on a tablet changes everything.

It is hard to eyeball it, so either you carefully construct the vanishing point for each recess, or you model this in a 3d program like blender.

I think you were sucked into he details. When rendering hair, start with the big mass, then add some locks to make it look hairy, and finally add some loose strands.

Hey Chrystal, Good job working on the Loomis head. The thing I notice the most is that your lines do not wrap around the shape of the head. The skull is a 3D shape, so make sure you wrap the lines around it. I did a quick sketch to show you what I mean :)