It's called The Soul Lies Down by the_moonmoth -- nope, not me, alas! but that just means I get to read and enjoy it without all the hard work of writing. ;-)
This story is all about Dawn creating her own happy ending. I love the tragic end of Fin Amour because TEARS FEED MY POWER but I understand that extreme angst as an ending can be tough to swallow, so I fully endorse Dawn rewriting the universe in-story to give readers a happy ending. Really, what more could you want?
How about this: The prose is absolutely lovely and I truly believe that the_moonmoth is my brain-twin when it comes to symbolism.
So if you love Dawn and Keyness-centric plot and Buffy/Spike and poetic prose and happy endings...
...is anyone even still reading this as opposed to clicking through to the fanfic?