When I went to Alaska--land of fresh seafood, which I could eat forever and ever, amen--I kept myself somewhat under control by not having a calorie limit, but still keeping a rough calorie count. It's not always possible to be exact, but you can estimate some, and that keeps you mindful of what you do
eat. I don't, however, suggest staying in mega weight-loss mode whilst in Italy, unless these sorts of trips are common for you.
Gelato isn't worth it, though; you can get it state-side if you have the desire for it later, but it isn't much better than ice cream. Keep that in mind to keep temptation at bay.
Also, if you go to the Vatican, just outside there, in Rome proper, there's this
restaurant. If you're going to splurge, I suggest doing so there on the gnocchi with gorgonzola radicchio sauce. It was really reasonably priced; they also, when I was there, had some healthy options--I remember specifically seeing fish.
. . .I'm not helpful.