At many points in my life I have tried to do sketching and recognize my problems in your description of symbol drawing. Since you sound knowledgeable about it could you possibly recommend something that expands on that topic, and gives advice for learning to draw that is centered on that concept?
Learn to sculpt! For me, a few hours making faces from clay was all it took to teach my mind the mapping from 3d to 2d. This forces you to think from 2d to 3d by taking an image you have in your mind and forcing you to build 3d support for it.
Then, do still life sketches of what you sculpt, no matter how bad. This seemed to reinforce the other direction for me.
