How to stop bitting my nails?

Well i have a super bad habit of biting my nails, i don't mean to and its not when i'm nervous.. its all the time and i only care after it happens. my dad took me out to Walmart today and i bought miracle growth serum thing, and i started using it but the problem is i cant make them grow if I BITE THEM! so tips OTHER then the lemon juice trick because i have tried it so so many times and it doesn't work with me