Hello Nghia! Quite some progress you made over little time, keep it up [:
I like your studies a lot, you keep thinking about the structure, which is nice to see.
An overall tip what I could give is that instead of only studying from pictures, you can start studying from life.
While your traveling in the train/bus, sitting in a restaurant, or just ask a friend to pose from you. Trust me, by drawing from life often you will improve way quicker than only doing studies from pictures. Which is to say that by doing those studies, it isn't ''bad'' in any way, it is just a different approach and depends on what you want to learn from it (eg. brush economy, edge definition etc.). By life drawing you improve your raw craftsmanship.
However, by studying from life you can walk around your subject, (sometimes) touch it and basically 'experience' your subject. It is from life, so it's 3D. A study from a picture is 2D, thus flat and static, you have to rely on another 'camera', instead of your own eyes.
I hope my long and boring talk gave you some insight. If not, shoot me a message or just ask away 
EDIT:
Potato language mistakes
