I'm a lifelong nail-biter, but currently "in remission" . I'm 65 now and started biting my nails when I was 5, when I started school and got told off by the teacher for sucking my thumb. Somehow nail-biting was more socially acceptable.
I've stopped several times in my life - when I was getting married, when my 4th child was born, when I reached 40 (and 50 and 60!). Each time I stop it feels like I've conquered it, but then every time when for various reasons my nails have been weakened (by DIY, gardening or some such thing) and start breaking badly, I am vulnerable to restarting and then get disheartened when they're all chewed off and it takes a while (sometimes years) before I achieve stopping again.
I genuinely do think that I have stopped for good this time - but time will tell!
The best ways that I've found for stopping are:
a) take photos of your chewed nails as a baseline, then take similar photos once a week to compare and see progress, which happens remarkably quickly.
b) tell everyone. Show all your friends and family your horrible nails. Let them see all the chewed stumps, the shredded cuticles, all of the things you don't usually want people to see. Then tell them that you're determined to stop and if they see you with your fingers in your mouth, they must pull you up on it and remind you you've stopped.
c) get some flesh coloured micropore tape and put a thin strip across each of your nails, at the top of the nail. If you inadvertently go to bite the nail, the unfamiliar texture of the tape will alert you as to what you're doing and give you an opportunity to choose not to bite.
d) after a couple of weeks, as soon as you can see some growth, file the rough ends smooth and put some clear, shiny nail strengthening varnish on. I swear by Nailtiques Formula 2, which works wonders on my nails. It's getting difficult to source nowadays and very expensive, but I think there are other Nailtiques fórmulas still available which may be just as effective.
Good luck! To encourage you, here are two photos of my nails. One taken at Christmas 2017 and the other one taken about 5 minutes ago. In between the two photos, I've stopped biting, then restarted, and then stopped again about a year ago!