Hi there! I think you seem to be doing at least quite well! You're proportions are ok, though I think you could practice them some more to really get them into your spine, for an example the face in the first one seems a bit too small and centered compared to the head, and in the top one of the other three you have almost the opposite problem. You seem to be fairly familiar with the anatomy of the head, except perhaps where the neck joins to the head, since you seem to almost be avoiding that area with heavy shading that makes the area look a bit too flat. You should really try to get the basic things correct before you go into the more detailed things such as shading and wrinkles, you're probably at the stage where practicing a lot gives you more than spending too much time getting finished pieces. Many of your problems probably boils down to that you simply need to draw more and by doing so practice both eye and hand. I really recommend to watch your drawings in a mirror, it makes it easier to spot mistakes that your brain has gotten "used to" while you draw and it often shows you where you're off in your drawing.Even looking at it in a hand mirror now and the as you draw is helpful.
About the 3/4 view, the last one seems better in perspective and placement of features. The first one seems to lack a bit depth in the features so that the nose seems to not go out from the face enough and the eyes seem to not be far enough in under the brow, if you can see what I mean. Both things look better in the second one. If you're drawing from life or picture then maybe you'd benefit from measuring the features and their placement with a pen or the like to really get it right. That way it's harder to get tricked by your own brain and simply draw what you see.
I hope that maybe some of this helped? I only learning to draw myself, and I hope you didn't find me to hard on you. Really I think drawing more is what will help you the most, but you're doing quite well so just keep going. Good luck!
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