I've been a nail biter my entire life. I have been utterly, finally and forevermore cured. Here is how the system works.
1. Move to a farm
2. Obtain Chickens
3. Obtain Horses
1. Feed chickens
2. Feed Horses. Make sure your horse feed requires hand mixing.
3. Clean chicken house. Make sure to get ALL the poop.
4. Groom horses. Also, visiting them in the barn throughout the day and giving really good scratches to their itchy butts helps in this process.
Now, for the final step. Forget one time...ONE TIME ....that you are now a farmer and even though you wash your hands and scrub your fingernails more than 5 times a day you will NEVER GET IT ALL!!!!
Nail biting cured.