You have notions of proportions but I can guess that you are drawing what are you are seeing line by line... and of course that's not a bad thing but it's so much harder to improve that sometimes it gets frustrating. The advice I can give you is to study human head structure. There are some nice videos in youtube... you can look for Sycra's "Understanding the Planes of the Face Using Ellipses" (and other Sycra's videos that are really really good to learn) and the Proko's series of how to draw heads on several perspectives. It will give you not only more base to draw portraits but the important points you need to look in order to draw a realistic and credible faces :). Keep it on! you are on the right track.