No one can teach you how to believe. It's like asking someone to teach you how to sneeze. It can be influenced but it cant be taught. For example, an advertisement is created to influence your belief in the efficacy of a beauty product. But its up to you to believe on whether what they're showing in the ad is true or not.
It's good to have doubts. It's always good to question, even your own beliefs. If you cannot believe that a spell can not bring you what you want, it is best avoided.