Is there a reason you don`t spend more time on drawings? Confidentially, if you spend hours a day, for years, you may get a handle on shading....it takes time to learn....of course it also depends on what you want to do, I have always said there is no wrong way to draw, everybody sees things differently, people achieve different levels in their work. Personally I am rarely satisfied, I always think I can do better but it`s all part of it, takes a lifetime to learn and I hope I never get to a point where I think I`ve learned it all.
But like I said before, making mistakes is a good thing, learn from them. I suggested earlier i this thread to practice with photographs, take a photocopy of one, graph it off, graph off some paper and take each square at a time, it really helped me learn a lot.