I used to bite/pick my nails so badly that I didn't even have nails. :Q I wanted to quit for a long time and I tried everything...nasty tasting stuff, band-aids, ect. Nothing worked. If I didn't bite them, I would pick at them instead. One year, lent came around and I decided that I would give up biting my nails for lent. I prayed, "Jesus, I want to stop biting my nails for you." So, for the next month, every time I wanted to bite/pick at my nails, I prayed instead. It was really tough for the first week because it was so automatic. Eventually, by the grace of God, I stopped!! I had destroyed my fingernails for the past 10 years of my life, and God allowed me to quit.
Hope my story helps.