Yo Onir. I also got a major issue with scratchy lines (or the feathered lines). Used to sketch lightly with a pencil, then go over the "proper" lines with a pen. Digitally, it's much easier... I just have a layer with all those messy lines.
Tried doing one continuous line (or rather, multiple continous lines) as opposed to short hesistant ones? Your lines show a lot more confidence in the master study though. And the latter ones already show a lot of improvement from the earlier studies.