Learning to draw, learning to see
Learning to draw is as much learning to see as it is anything else. You probably don't realize how little you actually look at something and see it for what it really is, to see the shapes and forms and lines that comprise it.
You have to re train your mind to actually look at and see what you are drawing. Not what you think you see, but what is really there. What the shapes are - what the relationships those shapes have to each other. One way to do this is to change what you are focusing on. Try copying a picture or drawing something from reference, but turn the reference upside down. This will force you to really look at and study the visual information you have instead of creating false information with your preconceptions. The more you can retrain yourself to draw waht you see instead of what you think the more truthful the drawing becomes.
Another method is working with negative space instead of positive space. Negative space is the space that the subject does not occupy, as opposed to the space it does, or positive space. Looking at the shapes and forms made up by the areas outside the subject, like the gap between the arm and the body or the spaces between the legs of a chair. By accurately mapping out the negative space, the contours of the positive space form automatically. Below is an optical illusion that illustrates how your mind by default focuses on positive space.
The image to the right has the negative space emphasized. The contours are the same, but you mind just sees a vase in the left and thinks, "vase." Without the internal detail of the vase you can see the faces in the negative space on the right. You see the forms, and not what your brain tells you.
When drawing from a model in reality, not a photo, try closing one eye when looking at the subject, since when you have both eyes open you are effectively seeing 2 images that your brain is compiling into one. This isn't necessary all the time but when mapping out proportions and angles it can have an effect.
The purpose of all of this is to make yourself think about what your are seeing. Drawing is all about proportion and relationships between elements. This isn't hard to do, it just takes some practice and willingness. Make sure you really look at what you are drawing. Unless you are drawing from your head, which is another story, but studies like this will help with all of that too.