Most Helpful Girl
She's still a kid, so call the parents! Tell them the situation and that she's not complying and they will fix her, doesn't matter how old you are, the parents always have that uncanny ability to revert you back to a child and do as your told.
If you have no way of doing this, state to her directly that things are OVER and that she has a certain amount of time to move out, otherwise you will take legal action.
If that doesn't work, then tell her again that things are over and that YOUR moving away, she's not coming with you, and again she has a certain amount of time to get out.
It sounds like you need to inform this girl with a bit of force (I don't mean physical) that things are well and truly over. You could even pack a few of her things into bags to emphasis the situation.
Also, you should move in with someone after you've been in a relationship for about 6 months, never less that that because, as you've found out, feelings change.
I wish you luck! Cause things like this are never easy.