pretty good stuff in here. Just always keep in mind your proportions when doing portraits. think in terms of negative space, how wide is the space from the edge of the nose to the outside of the cheek?, the bottom of the lips to the bottom of the chin? make sure it is exact and take your time. The slightest error could drastically change the apperance. Eventually you'll begin to automatically think in terms like that, and your speed will progress. These are some tips that have helped me.
Your doing exceptionally well and have inspired me to draw more. Keep it up.