I think my parents know that I self-harm, my dad threatened to "send me away" and my mom tells me not to "do anything stupid". What should I do?

If you are self-harming, you should seek help. Self-harm isn't something to be ashamed of. Talk to your parents If you feel comfortable enough. If not, a close friend. Talking about it helps and from there, you can get help and talk about what is causing you to self-harm.