<< I do it and can´t stop.perhaps 10% or more of all people do it? >>
Its common. I used to bite my fingernails something awful. It took me a long time to stop, with a real conscious effort. It was harder to stop biting my fingernails than it was to quit smoking, by far.
There is some stuff you can buy at pharmacies or get a prescription for that can help you stop biting your nails. It tastes nasty as hell and you brush it on your fingernails every day.