Who’s rules are you trying to live by? Your own? Your parents? The trend-setters in school? The unwritten rules of the society you are a part of, all the should’s and must’s as well as the should and must not’s?
Ask yourself. 
What is the rule?
How is it important to you? Why?
How does it affect you and the life you lead?
Is it a rule you agree with, or would you rather it didn’t exist?
Or would you simply rephrase it somehow? How?
Again. Awareness is the thing.
Awareness that there are rules, unwritten ones, that you have internalized during your upbringing, that can have a profound effect on your life.
So when you hear yourself tell yourself this cannot be done, that must not happen, oh no, you can’t wear that, don’t eat ice-cream for breakfast or whatever… ask yourself: why not?
Perhaps you will find a really good reason why not. Perfect. Perhaps you will find an equally good reason for doing it, despite the rule. Perfect. The point is to become aware. Not to act without reflecting, living a life by rote.
Ask.
Why not?