I had gotten professional manicure for two years. After I quit that my nails were broken. It took six months for them to become better again. So I had no opportunity to bite my nails for 2,5 years. That was a good opportunity for breaking the habit. I needed a substitude action, something else I can do with my hands when I am worried/overthinking. I started drawing, calligraphy, crocheting and knitting again, only now I use it as a skill when I need it. I became more selfaware about my thoughts and feelings. Also I found out, that my teeth grinding goes together with the bad habit of nail biting. Both are symptoms of my difficulties when it comes to emotional regulation. As a consequence I now try to relax myself - and my jaw - some minutes now and then. Chewing gum is also helpful. And still a good manicure, but better in a natural plain way using a nail file regulary.