lol, juvenile habits.
My brother had this problem about 3 years ago.
He said, and I quote; "I stopped when I put my mind to it that I didn't want to any longer"
It's simple as that.
When you notice your self doing it, stop.
Who cares if you're half way through a nail.
Stop.
EDIT: Jesus, that picture of your horrible nails stares into my soul.