Your dad isn't offering you any support. In a perfect world he would be able to understand and get you the help you want. You may have to be your own advocate and find someone to help you. Starting at school would be a good place. There should be a counselor that you can try and talk to. Beyond that maybe you can find the number to a local psychologist or therapist and explain your situation. Coming from a professional your needs may be better heard.

Self harm can be different for a lot of people. From what you describe you are really depressed and self harm may be a way to cope with how you feel. If you take that away by just stopping you are still left with the problems and without an effective means to deal with them. Therapy can teach you some skills to improve the quality of your life or help you sort out issues that are causing you distress.

Ignore your dad's ignorance that you can just stop and all of this is in your head. Your pain is real and your emotions are valid.

I'm not a therapist, but if you want someone to vent to or get some advice you can message me. I hope you are able to find the help you need and keep fighting for yourself.