I know what it's like, I suffered for a long time with nail biting. There is a lot of ways to help this problem, but first you must make a commit to stop. For me biting my nails was something, I started doing as a kid and just never stop, I tried to stop many times... but could hold to it. Until, recently.
These tips and advice really help me, I hope they help you!
1. Get fake nails, the limit the access to your real nails... Giving them a chance to grow.
2. Wear gloves, like getting fake nails the limit the access to your own real nails and in addition keep you from seeing your real nails. Which might be a trigger to start biting them.
3. Paint them with a nail polish with a bitter taste like "Thum Thum Anti-Thumb Sucking & Nail Biting Liquid." It doesn't have to be these brands, there are a number of bitter nail polish out there. They let you know when you put your nails in your mouth, so you can stop. Truth be told if you keep biting them though the bitter taste after abit it won't bother you, so stop when you notice!
4. Chew on something else like a Bamboo Skewer, but you don't want to eat them... of course. Just lightly chew on them. Or safer, yet. Try candy or gum.
5. Rubber band around your wrist to pluck every time your fingers are in your mouth.
6. Make a list of reasons you want to don't, they will help with you commit to stop.
Do you want to stop so you can paint your nails? Do you want to stop, because you realize the nasty germs, that live under you nails?
You must commit to stopping these things will only help, if you have the willpower to stop.
If the list method above don't work. And you feel as though the nail biting is out of control. The next thing you might want to do is seek help, you might have Onychophagia a form of OCD.
I wish you the best of luck,