Is there a way to know which levels I completed on hard?

[Leooon]

I changed mid campaign and now I'm lost

[Waste_of_skin]

Unfortunately, I don't think so. When they added the difficulty setting, they don't seem to have added anything to show which difficulties you have used on each map.

[Misfits]

It's ok if you didn't choose black character.But for getting that achive,sorry about that you have to play it all again

[February]

No, no indication for that.

There's only 3 things that show level completion: if you cleared it at all, if you cleared it on Heroic and if you cleared it on Iron difficulties. As far as the 'regular' difficulty goes (casual, normal, hard), that is not really tracked anywhere.