You're off to a good start. I certainly wouldn't rate you a mediocre artist, but rather a "talented student".
If I can make a suggestion, try to find a copy of "How to Draw Comics the Marvel Way". Even if you aren't interested in drawing Marvel style art, it's a great primer for proportions (which you seem to have a nice grasp of), perspective, using primitives, posing and inking style.
If you're more into a manga style, there are a number of good volumes available at most good bookstore chains.
Finally, if you're interested in working in the digital realm, especially coloring, check out the publications of Antarctic Press. Their sets even include a CD with the art discussed in the book to practice with.
Keep it up. We'll be interested in seeing where you go from here.