Can Anyone Explain To Me How I Can Shut Myself Down?

Lately everyone assumes I have been doing far better in my battles against self injury because they don't see me cutting. That's not the point, they can't see them because they are hidden, in truth it has actually gotten much worse. Lately in moments of stress, pressure, extreme emotional confrontation I have noticed I have the ability to make myself feel, for lack of a better word, "numb". For example, when my mother was yelling at me and I was almost in tears I told myself, commanded myself, not to let her get inside my head, and that's when I felt myself slipping, I was going to cry. Then I snapped, shut down, went, as I said "numb". I couldn't feel anything, emotional or physical. It seemed wonderful at the time but getting out of this state of mind took longer than getting into it and made me feel worse afterwards. It lead to cutting. Now I have learned to control the ability and am using it more often to detach myself from the emotional distress I feel at a moment in time, however I a not enjoying the side effects of feeling horrible and having to cut to regain more feeling. Can anyone explain to me what I'm doing to myself to shut down and if anyone else ever feels this way? It would be greatly appreciated, thank you.