Autistic children often bang their head because it releases endorphins (the "feel good" chemicals our brains naturally produce).

http://www.williams.edu/imput/synapse/pages/IB5.html

I've been impressed by my daughter-in-law and her treatment of my 3 year old granddaughter and 2 year old grandson and how they'll get to wailing and she'll say, "Use your words!" Maybe you could try substituting some of your self-mutiliation for journaling/writing it out or "talking" to someone?