Assume a virtue, if you have it not.
That monster, custom, who all sense doth eat,
Of habits devil, is angel yet in this.
– William Shakespeare
Hamlet, Act 3, Scene 4. Hamlet counsels his mother Gertrude to break a bad habit by replacing it with a good one. So pretend to be virtuous, even if you are not, he says. Habit can be a devil or an angel.