Learning to Believe Dreams

I really want to change the way I think about reaching my dreams. I love what Brooke says about believing the impossible. But when I try to believe the impossible things I want, my brain goes crazy with thoughts to disprove it. They usually involve evidence that is pretty logical. My brain wants logical evidence that my impossible dream is even right to dream. How do I stop this? Do I have to let go of the desire for any logic or evidence in believing?