I really enjoyed this movie. First let me explain why..
MAYJOR SPOILERS!!
so I think why it's called “I believe in unicorns” is because the girl is still a child (16 years old) she is still innocent, and naive She is the unicorn. Soon meeting the opposite a dragon, he first seems like a nice guy not like his father (his dad was abusive) soon he changed, just using her for her body and since the unicorn thinks the dragon (her boyfriend) is her "true love" she accepts. In the middle you can hear as they are talking about fairy's and dragons, basically metaphors as the dragon starts attacking the unicorn. (Divine and her bf is getting toxic for each other) soon her boyfriend took away all her innocence, and corrupted her. At the end the dragon kills the unicorn and the unicorn dies. (Meaning he took all her love and crushed it, and the unicorn gave up.) Soon you'll be seeing Divine as a arrow person shooting the dragon trying to save the unicorn but it's to late. (Her innocence, and childish self is gone. Shes no longer a child, but now a lady.)