What to do??

I'm 17 years old, and for as long as I can remember, I've been biting my nails. Well, a couple years ago, I started biting the skin on the tips of my fingers and eating it. I bite them so much that it hurts to touch anything. I've no idea why I do it, I wish I knew. It has made my fingers and fingernails looks so horrid. I try to not do it, but when I'm not thinking about it, it just happens. I get yelled at about it all the time. Would anyone might know why I do this?