I think it means that someone is having a dream about the person that they like/love, and it's so good that they don't want to wake up from the dream. & I think that she's always thinking about yesterday or the coming night because she can't wait to dream about him.
I'm just adding this part, but maybe she thinks one day it'll be reality, so she has to keep dreaming. She can't forget it because dreams come true, if you don't lose sight of them.