I know I posted on your profile already, but I hadn't seen your drawings yet. Now that I have I can see that you are off to a good start with your mid tones. Now go back and start filling in the darker areas by pressing harder with your pencil. You can use the side of the pencil lead to cover larger areas and then use the point to work on detail work like the eyes, tack, ears, etc. The nice thing about using the side of the pencil over larger areas is that it in turn sharpens the pencil for you.
You can use an eraser to mark the highlighted areas or as I often do, just leave the highlighted areas alone and work around them with the pencil. Having many different tones (aka darkness vs. light) makes for a more 3 dimensional looking drawing. I used to be cautious with darker tones, but overcame that and just went for it. I definately had failures followed by breakthroughs. Because I have no formal training, i have relied on taking chances and learning from them.
Good luck. Can't wait to see how you progress.