There are stopgap measures (worry beads, fiddling w. hair, evil-tasting polish) etc. but the root cause is important for you to discover. Nail biting is one common way of relieving anxiety; overeating, overexercising, cuticle chewing, pulling out hair or eyebrows, facial tics, and more draconian – cutting, anorexia, bulemia, morbd obesity etc.
We all feel some anxiety; severe nail biting is really a form of self-mutilation (and will turn off a lot of women…or men…. who like men w. attractive hands – I don’t know whether that matters to you.)
Some short-term therapy might help you to figure out your issues and also a way to substitute some more healthy and attractive way of staying calm—- the classic ones are exercise, meditation, yoga, tai chi chi gong and other eastern Martial Arts, diaphragmatic breathing, massage, hypnotherapy, etc.
I was a nibbler as a teen-ager as were my kids. I grew out of my nailbiting and now love my hands. I restrained myself (w. huge difficulty) from nagging my kids and they too grew out of it….we all have had therapy at one time or another (not for the nail biting but for our anxieties.)
Good luck. Chew on a pencil, gum or toothpick while you start to be more introspective.