I think you should pick up a book on perspective, a good one I own is Perspective! For Comic Book Artists by David Chelsea. You should also think your light source(s) more. I think your problems are fundamental though, so you should pick up some fundamental books too and study some more from life, mainly still life. You're young, get the tedious stuff out (though to be honest, I enjoy studying, but it can be a bit over-whelming at times) of the way so you can be creative and know what you're doing!