You are asking for criticism, and I am going to give it. Offending you is not my intention.
Read this (I never heard of this website before, but my views are summed up in this article):
http://www.livescience.com/19878-drawing-ability.htmlAnd then this:
http://en.wikipedia.org/wiki/Pattern_recognition_%28psychology%29I can't find any links further explaining these theories. However, by now you probably understand what I am trying to say.
You are drawing what your mind is recognizing when it sees objects. You are not supposed to do this. Draw what your eyes see.
Personally, I have never had this issue. When I was a child, I would "practice art" by simply observing objects. I would take in as much information as I could, then create a three-dimensional rendition in my mind. When I had finally accumulated enough objects (in my mind), I would grab a piece of paper and combine them all into a scene.
I noticed that no one else was capable of doing this. I'd always suggest, "Draw/paint what you see, not what you think." But nobody ever seemed to get the true meaning of what I said, and so I'm providing these links, hoping you might understand.
There are so many processes going on in your brain that, when you finally "see" an image, it has been distorted into a lossy concept of what it really was. First you see it with your eyes, which already have distorted it by registering distance, light, and movement. Second, your ability to remember information depends on if you're attending to it in the first place, which you are likely
not, without even realizing. Then it goes through pattern recognition, at which point it is warped completely. It is stored as information, which then you try to retrieve (or more likely salvage).
Basically, if you want to draw realistically, you want to avoid losing information. Absorb as much as you can. Avoid losing as much as you can.
For shading, regard how light strikes objects and how perspective plays into this. Consider multiple light sources or, in the case of one, how a shadow is "cast" (it is actually the absence of light, remember). Never underestimate proportions and angles. Check your work in a mirror: flip it around, upside down, and correct for anything before it becomes too late.