Here's a fact that may help... nail biters end up losing more teeth to rot and infection than non-nail biters. Forget where I read it, but it makes sense. That and the germ thing was enough to make me at least think about not biting!
I bit my nails for over 40 years, from the time I first HAD teeth. Nothing worked, absolutley nothing. Then nail tips came out... I applied those, made my hands look nice, and although in the beginning I did rip off the fakes, slowly I stopped. Now that I do a lot of typing, my nails are nice and strong... I have to use toenail clippers to cut them back! Carrying a nail file and rounding them nicely helps, as well as doing whatever I can to keep my fingers busy doing something else.
Good luck!