How To Control Aggressiveness And Improve Self Confidence?

i have extremely low self confidence, this always leads to me making little mistakes which make me seriously angry. This results in me self harming, hitting myself, pulling out my hair and shouting at people. This then makes me worse and it s a constant cycle of anger and the idea that I m terrible at everything.It s starting to effect the relationship that I m in and I seriously want some form of diagnosis so I can know if I need help.