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You should go with cause and effect tactic. Rather than letting the characters move to the way you desired you should imagine yourselves as the character you made. What the character will likely do and how will she react base on their personality and traits upon the given circumstance? This means you need to understand the character you create. The best way is (if you haven't done it that is) character building. Write the character's background. Is she a nobleborn, a commoner, a part of some tribe, or something ells? Where did she live, snowy mountain where food is lacking or surrounded by forest blessed with bountiful natural resource, or scorching hot tropical area near the sea? What is her relationship and interaction with her family, friends, neighbours, and those around hers. Are they on friendly terms, in conflicts, awkward situation? How is she being raised? Pampered, strict, neglected? What is her moral value? Kindness, justice, freedom, or love? What is her greatest strength and weakness? What is her flaws and how will she overcomes it and become a better person? As you think about these carefully, the story itself will naturally come to you. Moreover, it will make your character into more of an actual living being. Not just the main character, but also the important character around her like her family, friend, and the antagonist. You don't have to do everything all once, just one step at a time. Just start from what you think is necessary.
The nature of her power is also something need to be considered. What is its origin? Bestowed from heaven? Gift from mother of nature? What is it that make it so priceless?