Learn perspective, it will seriously improve how you approach environments and drawing people/ objects. There are some good tutorials on ctrlpaint for informal perspective and many good books out there on formal perspective, such as David Chelsea's book 'Perspective! For comic book artists'.
Anatomy also needs major work, before launching yourself into painting skulls, heads and torsos, learn to see them in simple planes first, learn basic proportions. Get a solid grounding and practice as much as you can from life.
In master studies, attempt to get all the measurements accurate first, before moving onto details. Simplify.
Overall, it looks like you're working hard and doing well, keep going
art blog: http://hrartwork.blogspot.co.uk/
"Don't worry about your originality. You could not get rid of it even if you wanted to. It will stick to you and show you up for better or worse in spite of all you or anyone else can do" Robert Henri