Every time my girlfriend and I got into some sort of a fight, I would get easily mad about everything. I don't want to hurt anyone, especially her so I would just bite my arm to hurt myself and only feel pain and forget about the anger. Sometimes I would hit my head in a wall, most of the times I imagine killing people who are close to me and I actually find it interesting and it scares me. What should I do? By the way, sometimes I get really angry or really sad for no particular reason. Why?