Why do I keel dreaming about my ex girlfriend!!?

Me and my ex girlfriend broke up 3 weeks ago. we dated for 13 months. Ever since we broke up I just cant stop dreaming about her. I always dream about us getting back together. And it hurts so bad because its reality. I just dont know what to do, I am so sad. And these dreams make it worse. How do I move on if Im dreaming about her everynight?