I stopped banging my head due to a neck injury and also the knowledge that repeated mild head trauma causes Parkinson's disease and dementia, both of which frighten me.

The only recourse I have when all distractions fail (including going out so my actions would be visible) is to phone someone, usually the Samaritans which is the uk crisis helpline and they often help me to calm down and avoid self injury of any sort.

I have taken to substitute other things, including hitting other parts of my body if I really Can't resist, not as satisfying, but the injury is less serious.

I wish you luck and the ability to find a way to help yourself avoid serious consequences.