Well I don't like copying either, but that's not the point
The point is that you observe how the shading will fall, and it will stick to your brain over time... No one says -all- of your work has to be original. Specially when you just do studies.
I tend to get frustrated too.. But lately I've just been trying hard, and tried to ignore my failures... As I wanna learn, not create amazing pieces of art. I realized that pretty much the only way to learn is to make mistakes and learn from them. I mean, I'll be trying the same thing a couple of times untill I get it right. Trial and error.