Yeah, thinking 3d while drawing is very important.
You have to know where the corners of forms are in order to turn them in space. Having a keen understanding of figures in their simple forms is also important.
Animators also have the ability to stretch and squash those forms while at the same time keeping the same volume.
I remember taking tennis lessons. I'd go on the ball machine or hit forehands/backhands against the wall. Over and over and only by chance I'd get one good shot in. After awhile I could hit that ball anywhere I wanted from anywhere on the court but it took some time. It's the same thing.
Say a Preston Blair lesson requires that I first create a sphere - it always comes out slanty, or pear-shaped, or ovaloid, or some other mess. I've tried drawing circle after circle on page after page, and only by chance do I ever get a perfect sphere -- and this is just one kind of shape!
You'll have to keep practicing on that control of pencil. It's not gonna happen overnight. Just keep filling up those pages with circles, different sizes, going from elipses to circles and back (there's a exercise in Vilppu's book like that). Practice circles going left to right and right to left.
You can even practice just drawing lines with confidence. Straights, curves, S curves, parallel lines. Thin to thick and thick to thin. You're not trying to draw anything - just developing that hand to brain connection, forging those new neural pathways.
Draw squares, boxes, any of those simple shapes. But practice with confidence...and use your whole arm too. Then you can start working those confident lines into more complicated things like ... the flour sack!
How do you hold your pencil, like you're writing something? Hope not, that's very restrictive. Remember, the whole arm!
Like I said, it won't be over night but if you keep at it, you'll start to notice improvement and soon you'll be putting those spheres down like a pro!
You are a level 8 ninja and even though you have a lot of weapons sometimes your ninja moves are your most powerful.