This is from the HD Holiday exchange (hd_holidays). Pastiche Pen came across it last week and shared it with her fellow HPDM appreciators. I am so happy she did because this is easily one of the best fics I've ever read, in any fandom. The writing is stellar, the story is excellent, the characterizations are just so good.
It's a novella although it's all posted on one page, so you can sort of just keep reading and reading and reading...
Fic details from from the hd_holidays page:
Title: Says The Magpie To The Morning (Sorrow, Take Your Own Advice)
Pairing(s): Harry/Draco (with just a smidge of Draco/Blaise)
Summary: It's terribly bad form, sleeping with your ex when you're still half in love with the bastard.
Epilogue Compliant?: EWE (Epilogue, What Epilogue? for the HP newbies.)
Word Count: ~34,000
Story link: Says the Magpie to the Morning