Shame
Shame is the most toxic aspect of BFRBs such as hair pulling and skin picking. Healing shame and becoming more resilient to shame may not be as visible as new hair growth or healed skin, but it is an essential part of treatment and recovery. Shame is universal, operating deep within our limbic system. Brene Brown has many books, videos, and audio series to discuss shame and vulnerability in great depth.
http://www.ted.com/talks/brene_brown_listening_to_shame.html