My therapist told me recently about the psychology behind self injury.

She said we feel like a victim and subconsciously are trying to hurt the abuser/bully by hurting ourselves.

I could see what she meant after she went into detail.

Her advice was; don't be a victim, don't let the abuser/bully win.

This is just one therapist opinion, a very high percentage of people who S/I were abused in some way.