Sorry that i have to use English - my chinese input app stoped working, I still can't fix it...about improve your drawing...I saw your watercolors and knew that your can draw. It seems to me you have a great understanding of structure and space, which is enough. But if you want to develope you skills in the drawing as a media/displine, then you need to have a deeper understanding of the marks you are making and the surface/composition you are creating.
Let me explain: anyone can draw, because a drawing is made with marks. Even a money/elephant can put marks on a sheet of paper, but only an artist knows "why". Why this mark, why this mark is here, is it working with the whole image? why this mark is made this way? does it say who i am as an artist? does it help me to say what i want to say in my image?....
Surface and composition, why this texture? why this ratio between light and dark? are there interactions between each elements? are these kind of interactions what i want for my image?....ask yourself many questions.
Before I paint, I have these conversations with myself. I often stop and look at what I have on the canvas while painting and have these conversation again and again....till I finish, then I ask, is this really finished...what can i do without? is there anything missing? It is a reflective process for me and I loath it when people decide I was just copying a photo.
Thank you for your encouragement and I hope I have answsered your question.