This sounds like a habit caused by stress. I have found that the easiest way to break a habit is to replace it with something else. For instance, every time you get that feeling that you are about to start picking your nails, try rubbing your hands together instead. You should also keep nail clippers with you, but don’t clip them in public, that’s not appropriate. If you keep your nails clipped, look at them and tell yourself often that they look nice. Soon you’ll get used to them looking nice and you won’t want to pick at them. Growing up I was a nail biter. At the age of 18, a girl friend of mine got tired of looking at my nasty nails and started giving me manicures and she would massage my hands with lotion to keep them soft. I got used to that real quick and stopped bitting my nails. She would always tell me how nice my nails looked and so would others. My nails have been bite free for 26 years now. Good luck.
