Sketching was always the most difficult part of drawings for me. Work on the other stages, like lines, shadows, or shading, is just boring and exhausting, but still simple, because you know the whole amount of work and where should be the line. Sketch - is the totally different... this stage you always have to draw from the zero, taking all from your head, if that doesn’t help - use a lot of different references, always looking to them, to be sure that everything was made right on your sketch. Proper proportions, clearly visible elements, correct angle, whole composition, etc.
This piece made me a nervous wreck, while I worked on it, but I'm glad anyway, that this sketching stage for this art is over.
Next stage, the linework will be a bit boring, if you count the amount of details, but this is better than first stage, because now you know the full amount of work, and how this work is supposed to be done.