Now that we've laid it out on the table for you, it's time to get real with yourself. Which of the aforementioned categories apply to your situation? Have you just chased her away with your antics, or has she fallen out of love?

The unlikely good news: If it was you fucking up, you have a far better chance of reuniting. Just be warned, there will be significant effort involved. Your first plan of action is acknowledging your mistakes, and expressing regret. Think (really contemplate!) how your behavior has affected her. Let her know you understand the role you played in the relationship's demise, and express sincere, painful regret. 

If you're in the latter camp, and your girlfriend has said things like, "I'm not sure what I want anymore," or "I just need some time by myself to figure things out," she's likely questioning her own feelings, not yours. The appropriate (and most effective) response here is to tell her you understand her feelings, rather than try to talk her out of them.