Then Tomorrow Came is a My Little Pony: Friendship Is Magic Fanfic by Ponyamorous. It is currently complete and can be viewed here. The sequel Heart of Loyalty is in progress and contains major spoilers for the ending.
Twilight has a problem. Despite her longstanding opinion of "love" as an affliction that turns ponies into simpering heaps of irrationality, she is reluctantly forced to admit that she has become hopelessly smitten with Rainbow Dash. After discovering that love hasn't turned her brain into mush, she prepares for her big confession, only to be blindsided with the news that Rainbow Dash and Applejack are now together (and quite happy). She tries to accept the fact and move on, but discovers that such feelings don't fade easily. She finds herself wondering just how long the pain is supposed to last, and what 'moving on' even means.
This fic contains examples of:
- Dark Fic: A darker ship fic covering the pain of unrequited love.
- Green-Eyed Monster: Resisted by Twilight, who despite her envy of Applejack, recognizes that she's still a good pony and doesn't bear her any ill will personally. The occasional graphic fantasy to the contrary.
- I Want My Beloved to Be Happy: Twilight deliberately attempts to invoke this, but finds that it simply isn't enough to love Rainbow Dash from afar without being involved directly.
- Love Triangle: Type Four, with Twilight on the losing end.
- Unrequited Love Lasts Forever: What Twilight fears may be the case.