I used to bite my fingernails a lot when I was younger into my teens, although not to your extent. I guess I was just fortunate to "grow out of it".

I believe you can get a sort of transparent nail varnish that is meant to dissuade nail biting.

I have no idea whether such things work, but it's probably worth a shot.

I definitely agree with Skeezyks that having a nail clipper to hand is a good plan too.