When people ask me “how do I learn portraiture and composition”, I always have a simple answer. Study what others have done. In particular, study classical painters. Go to an art museum!
Look at the work of Rembrandt and Vermeer. Velasquez. Or more recently, John Singer Sargent, a genius of portraiture. Or the impressionists. These people knew portraiture, they knew light, they knew composition.
And learn from photographers whose work you like. Avedon, maybe, or any of the other greats. This is the way to learn. Try to figure out exactly why you like certain works.
Another tip: ask yourself “what feeling does my photo represent”. If you do not know, chances are that the image is not optimal.
Using these learning opportunities, chances are that you will improve your work soon.