What happens when I say, "Don't think about a purple elephant?"
Yep, you probably thought about a purple elephant. 
What do you think happens when you tell yourself, "I can't eat after 7pm" 
Yep, all you think about is what you want to eat.
When creating goals for yourself, I talk about what types of goals to avoid and a better approach to creating goals that your mind is willing, open, and excited to achieve.