I had a really bad habit of this as a child and into my 20's. It was so bad that my parents actually bought this clear nail polish that tasted terrible to try to get me off the habit. Once I ran out of nail I started chewing on the skin as well.

I didn't do anything to stop and the bad tasting nail polish didn't work. I do not bite my nails at all anymore and I don't know exactly when that stopped, I think it was somewhere in my mid to late 20's. Hopefully for you it will be one of those things that just switches off one day like it was for me.