Exellent job -- such subtle and perceptive portraits of both of them. I don't see a lot of Slughorn fic, but ironically, almost every Horace story I've read has been exceptionally strong, and this one is no different.
I agree with kinky_kneazle about how effective it is that both of them have at least a moment of realizing the nature of their failures -- so humanizing, and in the best of ways: the two of them humbled and raw. The perfect description; it leaves them believably cleansed and ready for what comes next. Their backstory, too makes such sense, both in past and present.