As a child you didn't need proof to believe.  Santa was real, your parents knew everything, your body was awesome.

When did the doubt start to creep in?  Where did that innocence of childhood go?

Somewhere along the way you learned that your body can't be trusted, therefore you must rely on an external set of rules.  Eat these foods, but not others.  Count the calories/points.  Exercise for a certain amount of time, at a certain intensity, for a certain number of days per week.  When you can't follow these rules, how do you feel? 

Listen to your body's internal compass, it's there to guide you.  The doubt you may experience is normal.  After all, you were taught to doubt.  But after a period of trusting you will rediscover faith in yourself, and that's a beautiful thing.