People just don't understand...

I'm 15 years old and for about 10 years of my life i've picked and bit the skin around my fingers until they bled. My fingers are swollen and always have open cuts on them. Even though my fingers probably aren't the most visible place it's still as embarrassing for me if people ask why my fingers are sore. How am i supposed to reply to them? It's a form of self harm and they'd probably class me as weird. My family and friends don't understand they tell me to 'just stop' but what they don't get is if i could just stop then i would but i can't! They shout at me when they see the mess my fingers are in and i know it's all for a good reason but the more they shout at me to stop the more i'm under pressure to try and stop but then the pressure just becomes too much and i pick again. I don't know why i do it and i don't know why i can't stop and i hate myself for that.....