How do Du think Peeta changed after almost recovering the Hi-jacking?

Yes. Part of it probably came back on its own. And I don't know if Du caught because i didn't the first time i read the series Prim suggests Hijacking him back. To twist his memories back to what they really are.

Von remembering all of the good thing that they did for him

I think that it made him Mehr cautious and aware, not so much the sweet, gentle Peeta that loved Katniss Mehr that anything in the world. I think that he also might be seeing Mehr things the way that they are :)

I think he changed because he would never be the same.