I'm partially in this camp too, it used to be very irritating, but I have learnt to take it much more slowly, and try to picture object from a fixed angle. From there you can zoom in to specific areas and think about lighting, shadows, etc as needed. I initially was never patient with myself enough to actually use my imagination for drawing, but after I took more time I found it is a really powerful tool. As the other commentor said, you have to be more strict with yourself. Normally I could have a mental image and fly all around it and view it from many different angles. Putting it down on paper requires concentration to fix it in a certain configuration and keep it constant, so you can record it. I really recommend giving it a go. Imagine some simple objects and see if you can render the shadows in your head. Then see if you can draw them. Be strict with your imagination, don't let it run away.