score:12
If you are an Italian citizen, you can forget about all those Visa and stamps for Schengen - you are now a Schengen area citizen.
In the future, when you travel into the Schengen area, you should always use your Italian passport (or ID). Nobody will care about your 'never-ended' trip with the Brazilian passport, and if someone asks, you just tell them you became a citizen (and show your Italian passport) - end of discussion.
The only issue would be if you became a citizen only after the six month on the Brazilian passport / visa run out - but you said that is not the case, so you are good.
Upvote:0
Since proof of citizenship means you can't be penalized for an overstay, getting an exit stamp is not necessary. But if you really want one badly enough, maybe buy a cheap plane ticket to U.K., get the stamp, then go back out without getting on the plane. Use your proof of citizenship to avoid getting another entry stamp.
I don't know whether this would actually work, but …