Why don't you start with more traditional techniques instead and go from there? You look like you need a brush up on certain basics that you won't necessarily learn here.
Go make a few studies from life, using pencil or whatever. Get a hold of an art teacher and talk to him/her.
You mean I need to stay out of digital for now? Ok, got it.
Not sure about the art teacher part, however. Several major issues prevent me from executing this part of your advice: the fact that all available art classes in the local area are for children, the fact that the only art school in my hometown is for children (and I'm too old to apply), the incredibly busy schedule at my school leaving very few time for any possible art courses, etc.
A formal art teacher is not entirely necessary, but some form of guidance usually helps very much.
Digital painting is often something a lot of people try to do without the necessary experience generally speaking, thinking that if they go out and buy a fancy tablet it'll somehow make up for their shitty drawing skills. The thing is, it won't, and they'll fail repeatedly until they gain that basic experience somehow.
The method of just picking up a pencil and "going out there", so to speak, seems to be underrated. That's how a lot of art was done and is still being done today, even in those fancy art schools. Learn anatomy from drawing real people, shading from basic lighting studies and so forth - practice and first-hand experience helps immensely, and digital art discourages the use of real-life references by the simple fact that instead of looking at the world like you should be doing (since, after all, you're trying to render the world), you're sitting cooped up indoors behind a shitty artificial computer screen, plastic stylus in hand or what-have-you.
Hell, pick up some drawing/painting books from the bookstore and read those. Pick up some watercolours if you want and try that.
As for the current situation, I agree with Soadreqm's assessment that you're lacking in anatomy. However, I'd like to build on to that by saying that you should additionally be drawing with real references until you get an intuitive grasp of it (I'm currently pursuing anatomy studies too, and my teachers have all encouraged me to do real-life studies as much as possible). This does not necessarily have to entail finished pieces, shading, composition and all that - just make a bunch of sketches.
While you do that, it'll help to study existing works and various guides on anatomy - though you can certainly discover much of their advice by yourself, having a jumping off point works too - just don't substitute reading for actual, DIY work and study. Here are some good starting guides for anatomy:
1,
2,
3,
4,
5 (a collection of books).
You also probably want to take your work somewhere with more professional artists.
ConceptArt.org can help you with that.