My fingernail biting and other, similar issues got better with a lot of therapy for my anxiety. The problem lessened and then more-or-less went away with other bothersome symptoms.

I still bite or tear my fingernails when they get too long or split/break, etc. but I am much happier that they are in better shape most of the time. One thing I figured out for myself is that I bite them (and engage in other "automatic" behaviors) at particular times; if I have been drinking and/or doing reading for multiple hours, it can be bye-bye fingernails. The lessening of my consciousness and/or my engagement in other "worlds" that can be triggering or scary to me without my thinking about or realizing it, gets the unconscious riled up? Thinking about that now, I could put gloves on or something when I am doing those behaviors or otherwise make it so I am paying attention to my poor hands.

I assume you pick when you are alone, maybe studying or reading or watching TV, working on the computer, etc.? I would decide to give myself an acne treatment of some kind then; even just put cold cream or something else "messy" on my face so when I unconsciously started picking I'd feel the goo and that would give me pause?