I've been a nail biter for basically all my life, and I've tried several many times to stop. I think I'm finally on the path to finally quit the habit, and while I want to share less about myself for complicated reasons, I wanted to write something here in case it's helpful for anyone else.
For me, the latest attempt was spurred by wanting to learn to play string instruments with my nails. I don't know if having a motivating factor like that is essential, but it helped me. Maybe you'd like to pick up classical guitar, clawhammer banjo or something to that end. Maybe you want to pick up nail art. Maybe you just wanna stop biting your nails!
I've tried chewing gum, the bitter stuff that goes on your nails, bandaging my finger tips, gloves, etc.
In the end, I think these are maybe fine solutions for someone, but they were not helpful for me. The turn around for me was, while I bit my nails, I stopped, and thought about exactly why I bit my nails in the first place.
For you this might be any number of reasons. For me it was when something is out of place, I'm compelled to pick at it or get rid of it. If I get a stiff facial hair, I'll fidget with it until I finally shave or pluck it with tweezers. See more icky adjacent things like: scabs, pimples, etc. If it's a thing that wasn't there, I would like it to not be there.
Any time I'd grow my nails out, they'd get maybe an eighth of an inch out, and soon I'd be fidgeting with my nails.
Anecdotal stuff here: I'm also on ADHD meds, and I think that makes that much worse. I go from not focused to hyper focused, these go from molehills to mountains when that kicks in. I don't think that makes it insurmountable though. Again, just sharing in case you're in the same boat.
So I went back to the same initial realization of wanting to get rid of things out of place, and I did a few things:
First: I'd replace the behavior. This is not a cure all, but I suspect it's good thing to do for any behavior you don't like. I saw someone who picked at their skin too much when they talked on a podcast, so they replaced that behavior with playing with play-dough. For me I started moisturizing and rubbing my hands and brushing jojoba oil (keeps your nails from getting frail and brittle) on my nails with cosmetic twist pens. Good things to do anyways!
Second: I'm really not used to having fingernails, so when things would bother me about them, I'd address them instead of getting rid of my nails or biting them. Sharp corners? File them down. Something about my thumbs compels me to press hard into them. Filing down that spot down got rid of that. My pinky nails are the most egregious nails that I can't stop clicking with my thumb, so I just trimmed those down, and left the rest. This all worked for me.
Third: I grew my nails out as much as I could stand, and then ended up trimming my nails down almost entirely. Maybe down to less than 1/16th of an inch of white so I still had nails. Just a little bit of white. This is not normal for me.
That sounds like throwing in the towel, but I know me, and I know if I hadn't, the anxiety of them simply being there was getting to the point where I one stressful moment from chewing them off. It wasn't conceding defeat, it was a tactical retreat, and I didn't beat myself up over it. I'm conditioning myself little by little to have my nails feel normal.
My nails are already close to where they were last time I trimmed them, and they bother me substantially less now than they did then. My pinky nails are still shorter, but they bother me less and less too.
I hesitate to diagnose myself, but I'm pretty sure whatever neurosis I have that compels me to bite my nails is here to stay. I'm not there yet, and more trims are almost certainly in the future, but whatever brain quirk I have, I think I'm on the path to condition it little by little to forget I have longer nails at all.
The cat has gotten noticeably happier to get scritches now too, so that's nice!
I'm not an expert of whatever all this is, but I imagine it's translates well into other behaviors you might want to stop. I at least hope this helps people that want to stop nail biting.