I have had the same issue, and your question how to stop biting your nails was once mine. It’s really frustrating, but I somehow managed to overcome it. Look at your nail biting in the same way you look at your eating habits. The nails cannot be so appetizing that you cannot help but bite them constantly. So I put some chili powder, lemon or other distasteful substance on the nails. Whenever I put the nail in my mouth, I immediately spit it back out. After a few times, I was fed up and quit nibbling on my nails. Once you’ve done it, it never felt that hard any more. So I suggest you try this trick too.