Wanted to second the comment above. You write a Horace Slughorn that is believable, fallible and one who has depth of feeling to match his ambitions. Written in this way I actually liked the man and was rooting for his relationship/courting to succeed.
I also like your Severus. His edges are a little bludgeoned, he's not quite as prickly as he was, but you write the ravages we might see if we too could see under that glamour (which is lovely by the way because you hint at the man he might have been without 20 years being tortured as a spy and keep true to the old adage that you can never escape yourself.)
The hope for a happier future comes from the fact - to my mind - that Slughorn can and always will be able to see through the glamour. He'll know why Severus has nightmares. And old Horace is far more pragamatic at being able to put aside guilt and regret or at least move forward in-spite of them.
Really lovely fic! Written with less deft and skill, and with such a pairing, this would utterly fail to work.