I think you have to concentrate mire on the purpose of the story so that when you get down to writing it it stays true to what it is supposed to be. It is not about and ordinary girl with a prominent birthmark, it is about a girl whose life had been rocked and permanently altered, about her struggles to realign her life. The last few chapters could have been any person.
A story must have a purpose, a theme, an ultimate goal for the protagonist and the story is the journey through struggles and hurdles that conspire against her ultimate goal.