Your portrait looks good. And you have received some good advise on this thread. I just have a couple things to add.
1. How big is this drawing? It looks a bit cramped. Try getting a big sketch pad, like 18" x 24" (or bigger) to give yourself some room.
2. I second the recommendations to do some quick sketches (they're called gestures and they are good for understanding the movement and pose of the body) and also doing some anatomical drawings. They will really help you understand the structure, skin over muscle over bone.
3. Try drawing yourself with your hair pulled back into a bun or ponytail. The hair looks great, but it will help you improve the portrait if you can see your face and bone structure more clearly.
Keep up the good work and don't be afraid to take chances. I look forward to seeing more of your work!