Hmmm if you can't copy basic shapes like squares or circles you shouldn't be able to copy more complex shapes like spheres or parallelograms.
Awww crap I've just lost so wonderful inspiring speech typed here due to unintended refresh of the page.
But it's so good that I'll retype it for you.
The skill that I am talking about isn't about reconstructing shapes from primitives. No, it's not about those famous draw-an-owl-from-circles technique. It is about lines.
That skill is an ability to see lines. Not shapes. Not bones, lines of action and theese sticky-sticky-cucumber things. It is the skill to see every line in the scene, see its very configuration, its every angle, its every curve, see exact boundaries of shapes without messing with that shape's meaning.
Essentially, all lines are of the same difficulty, because they should be processed in the same way, as plain curves. That is, it's not much difference in difficulties of drawing a piece of confetti and a maiden's fetishized foot... held by fetishized hand... and bitten by fetishized mouth with as well fetishized milk white teeth. Both scenes are made of lines, and all those lines are of the same difficulty - you just pick them one-by-one and move to the paper.
Good half of the mentioned book teaches you to see lines. I don't have any will to type more and spoiler it, so you better get and read it yourself.
That was my opinion, though. However, reading the book surely won't be harmful... as I judge from your drawings. :3