What do you do with the pieces? Eat them? Throw them on the ground? I had this problem years ago (maybe not as severe as you, but also biting nails since childhood – which is a nervous habit) and I spoke to a rabbi about it. He told me that if I’m eating the nails, they have tumah and that I’m putting the tumah into my body. If you are leaving them on the floor, then there are issues with a pregnant woman walking over them. Maybe if you focus on what are the consequences in this world of what you are doing it will help give you a sufficient deterrent. It’s not always possible to worry about what’s going to happen after you die unless you are already ill with a terminal illness, since then death has more of a reality to it than something that will happen “eventually”. This finally got me to stop biting my nails.
Another thing would be to see if you can figure out why you do it? As I said above, it is usually a nervous habit. When you find yourself that you are already biting them without realizing it, as you put it? If you are doing it out of stress, see if there are ways to lower your stress level before it gets to the point that you start biting.
Hatzlacha finding a solution to your problem.