I've been looking for this for months to no avail.

The war was pretty much over, and there was only a small group of familiar names still alive. Harry was pretty much batshit crazy, from losing so many people that he loved + the residual effects of a curse.

He broke into the DoM to try and find something to end the war, and instead found a tome in the time room that he became obsessed with. He confided in Neville, who was pretty much the only other person still alive, that he was going to go back and save everyone and kill all the death eaters before they were a problem.

Neville was outwardly agreeable and supportive, but made plans to hitch a ride back in time with Harry, not trusting him to be effective, subtle, or sane.

Then there's a fairly lengthy story of Nevile working from the shadows to try and fix all the fuckups Harry causes.

I remember it being kind of hilarious, and thoroughly enjoying the Neville-centric aspect of things, but...I can't find it.

It was in the 150-200k word range when I read it at the time, but it's been a year or two, and who knows if it has been updated since then.

I remember nothing about the title or the author.

 

Wow that sounds really good. Posting so I remember to check back once someone links it xD