What should i do in the case of feeling really angry or upset and struggling to hold myself back from self-harming?

First thing, which one is already doing is recognizing the feeling and struggling to control it. The next is to get Counseling and Psychology Consult from professionals as soon as possible. Meanwhile sharing the challenges with a trusted Adult- Parent/ Teacher/ Mentor or so will help. It is commendable one is reaching out for help.

Seek medical help, and talk therapy. Prevention help lines are available on the internet and by phone call.