STAGGERING improvement here Leonard! You should be proud of yourself. Great discipline, great studies, and you are definitely studying the right things, which is most important of all. I see a lot of form studies, and your dedication has improved your drawings of figures really noticeably.Have you studied much perspective? I saw on an older image of yours that many of the cylinders had rugby ball edges, which is why I bring it up.
Some will stress that you should always have a "background awareness" of perspective as you draw forms. I bought Andrew Loomis's "Successful Drawing" book a few weeks ago and he really emphasizes that perspective is an illustrator's most crucial skill. It might be worth trying to find some of his pages on perspective, and studying it for a few days.
If a drawing is not working, instead of fiddling with it and getting frustrated, perspective is that extra tool of helping you figure out what's wong. Knowing the rules well means you can see which shapes are well-placed in your "imagined reality", and which ones you thought were well-placed but as you give them more attention you realize they weren't. So I highly recommend reading up on perspective, Loomis was great for me but I'm sure there are lots of good guides out there.
All the best! :]