Speaking as an ex-nailbiter, I offer these tips -
Most important, keep the cuticle pushed well back, This stops the red shiny skin at the nail base with those tempting little bits of skin sticking up, and instantly makes the nails look a bit longer.
It's all too easy to find your fingers in your mouth & you haven't even realised you're nibbling, so
Put on some nail varnish, possibly clear, so you bite that instead.
Dip your fingers in neat Dettol, the taste should make you take your fingers out of your mouth.
I have even heard of people walking round (at home) with a baby's dummy in their mouth, so they can't nibble!
Best of luck.