Don't Muck it Up (слэш)

This is an old Snarry, written possibly late 2006 or early 2007. I think I posted once on some site, but the I forgot about it. I didn't really like it, so I never revisited it until now. I've almost completely rewritten it in the last few days, so here's the new version.
An older Harry is sent back in time, by a meddling Dumbledore, to stop his younger self from making a monumental mistake. The question is: Will young Harry listen to reason, or will his future happiness fall by the wayside because of his stubbornness and refusal to let go of old grudges?