Drawing requires much looking, much getting to know why (in this particular case) the branches of the tree meander hither and thither through space, getting to understand what is going on. A drawing or a sketch doesn’t have to be perfectly accurate, this is not what looking or observation is about, no, what’s important is what you learn from looking and drawing, what you learn about the thing you are studying. (Afterthought: and possibly yourself too.) This is a pen and ink drawing / sketch using a new fountain pen just received postage free from China and it cost less than £4 sterling!!! But it’s just as good as the more expensive fountain pen I bought from Japan. And it’s much better than the constantly flooding Rotring Artpen.
peter robinson fine art 
This is good
I’ve been trying similar techniques and subject, but I like your results more.
I’ve now acquired several different kinds of pens… and ink and paper etc. but I haven’t got into large-scale production yet (one should be able to knock out at least four small studies a day) and the winter cold and wind and rain don’t help… I think once you get into the swing of it everything will fall into place… but you have to get the right pen… I’m currently waiting for a set of Rotring pens from Amazon… the Isographs not the Rapidographs… college set I think they call it… I once used Mecanorma pens but can’t find a dealer who sells them any more… maybe they went out of production… and ink washes, I haven’t got into them yet, but they really help… I used to begin with washes starting with the lightest and when dry go onto the pen and ink… just building it up slowly…