Taylor Swift - The Moment I Knew Lyrics Meaning

This song clearly translates into one where the girl goes through a hard time understanding what is keeping the boy from being there for here; not only physically but emotionally. She refers to the fact that "he said he'd be here" and "A million little shining stars had just aligned,
And I would've been so happy." She relates her being happy to his presence and that just merely seeing him would mean so much to here but the fact that he didn't show up makes her bottled up feelings explode and she's crying with her best friend there to help here thru it all.
She's in a party, her birthday most likely, and she's expecting for him to enter. Time drags on, every second, every minute, her face turns to the door hoping to see his tall figure illuminate the room. Every time it's someone else, disappointment runs through her and she can't hold it in anymore.
It's her day, she wants to have fun, she dressed up, had her hair done, got a great dress, Just. For. Him. To. See. But he didn't come, and that is what pains her. All expectations are crumbled to the ground.

And that was the moment when she knew that it was all over. Just over. Simply. Simply like that.