Hi Morphex. I understand how you are feeling. When there doesnt seem to be any improvement despite a lot of practice. But there are a few things that help me to keep going. The first thing that really helps me is to have some kind of routine. When I dont have a lot of time (right now I have a lot to do for university) I try to get up early and draw for thirty minutes. When I have a lot of time I write a plan for one week. eg. monday 7.30-8.00: practice straight lines and ellipses, 8.05-8.35:anatomy, 8.40-9.20figures in perspective.... Use a timer for your drawing sessions. I recommend 30 and than take a break for a few minutes I used to do 5. But I think I will do longer break in my next vacation. When timer stops take the break and dont continue even if you want to keep going, if you stop while its fun you will want to come back to drawing. If you feel stuck with something eg. the proportions of the face stop practicing that and go to something that is more exiting. There are many different fundamentals. You could study composition, perspective, construction (which is basically a subset of perspective), color, gesture, handcontrol (drawing straight lines, circles, ellipses etc), visual library (studying the anatomy of different things), and probably some more. In many online courses one thing seems to built on top of another, and thats true to some extend, but its also true that all topics "help" each other. You dont study proportions for a few months, and than your a master at it and than you move on. Its more like studying proportions for a few days or maybe for a week or two and than you move on to the next thing (if you have a lot of time you should definetely study multiple fundamentals simultanously). But you are not done with proportions, but you will go back to it again and again. Rotating through the different art skills, continuing to perfect them I hope this helps. Im by no means an expert, but I thought Id share what works for me.

What I see when I look at your drawing is a lack of edge width. There is edge width to the top part that is tilted up, but not on the bottom part it rests on when down. I did a quick draw over to show you how I would do it, I hope it helps :)