it's awesome to see you working hard to reach your goals. The bargue studies are especially nice, you have really good clean lines there. As for your line drawings in general; you might wanna try to switch up the line weight a bit more to emphazie the most important points. Bob Kato has a really good DVD that covers some good tips about drawing in lines and drawing in general. It's really about drawing the adult main head but a lot about what he says can be generalized to drawing anything. If you can get your hands on it I can really recommend it, but otherwise his message (or what i can remember of it) could basically be shortened into this:
* draw thorugh the forms, that is pretend that you're painting on top of a sphere for example - and really feel the form when you draw it (I think you've grasped this idea already)
* overlapping lines are great to show depth
* use your darkest values and thickest lines to draw focus.
* dont be afraid to be lose and sketchy outside the points of focus. sketchy things tend to push things back,
hope that this made some sense and keep it up, you're doing great