biting and picking at fingers.

I am in high school and constantly stressed I have major anxiety problems... Ever since seventh grade I've been picking at the skin around my nails and even biting it off... I don't know why I am always doing this.. but I can't seem to stop. I am so ashamed of my fingers and I get some pretty strange looks when I am picking at them. So... does anyone have any advice to help me out? I don't know of anyone else who does this.