I'm 31 and I've bitten my nails all my life.
There's been a few of occasions when I've tried to stop, the longest being for about 2 months.
I find I bite them without even thinking about it. The last time I gave up I started again while watching a movie. I was half way through the movie when I realised I was also half way through my nails. It wasn't scary and I wasn't stressed and I can't explain it.
The one thing I've realised is that I have to genuinely want to stop. But I don't want to, I enjoy it.
One of my nails is really nasty looking at the moment so I thought I'd give them all a breather. It's been over a week since I stopped and I'm finding it really easy. But I'll soon get sick of clipping them and having to scrape out the muck that accumulates under them and I'll be back to biting them.
You need to want to stop for yourself and not because someone tells you that you should. The bad tasting stuff that you paste on your nails can only act as a reminder not as a deterrent.
To take your mind off chewing on nails, carry around with you a plastic baggy containing a delectable assortment of staples, drawing-pins, washers, and broken hinges....Good luck.
Every time you wash your hands get a soft nail brush and scrub your nails gently under the whites. I don't know why this works, but it does. I think it's possibly because by doing this you're consciously taking care of your nails. Always put a couple of coats of nail polish on too - you'll soon see them grow.