So what would make you happy?
A flat stomach?
I don't think that would be enough, personally. Sure, you really want it. And you'll get it if you stay committed to that
goal. But happiness is about deeper things. I've achieved some pretty big physique goals I've set for myself over the years and though they were acutely pleasing... bigger things have led to my chronic happiness.
A large part of it is managing your focus.
By saying things like, "Will I be happy when I obtain my flat stomach," you're telling yourself that your happiness is dependent on the reflection in the mirror. That can be a skinny line to walk without a net considering the fact that our looks our predominantly genetically controlled and even if you do obtain that flat stomach, it may not be enough.
Again, managing your focus and expectations are critical.