It's when Emma finds her girlfriend's lifeless body in bed she eventually breaks. It's the final straw for how much she can take and she ends up being admitted to a psych ward. During that time she turns 18 and should therefore be free from the foster system. However, due to her catatonic state, the court orders for her to stay in state care.
After being cleared by doctors months later, she moves to a couple in a town called Storybrooke, where she has to try to get back on her feet and finish her senior year in high school in order for the court order to be terminated.
Her plan is to graduate and get the hell out of there but plans seem to slowly change when she finds herself embraced by the people around her in a way she never has been before. When she finds herself connecting with her English Lit. teacher in ways she never could have imagined. A connection in which she could find something she's always searched for.
Swanqueen story through and through