Good question, indeed if all you do is work from photo refs and never practice in drawing and painting from memory you could become a ''slave'' to the refrence.
To understand why certain areas are lighter or darker in values and whatnot, a good place to start would be the structure and anatomy of the head ( as your already doing) With focus on proportions.
Once you have a better understanding of the planes and basic skull
you'll start seeing where things peak or depress, overlapping forms ect.
second question: You can never get worse , only better!
how fast you improve depends on how often you apply these studies to your personal work.
I hope that made sense.