I have driven extensively in Europe on several occasions, including the centres of Rome, Paris and London in rush hour. The OP is from Boston. In my experience, if you are a fairly confident driver and are used to driving in rush-hour traffic in a major city and on congested highways, you will have no problem driving in Europe. You do need to read up on the rules of the road for each country. These are usually available online and include such things as general speed limit rules, traffic signal rules, right of priority at round-abouts, restricted travel areas (such as ZTLs), vehicle lighting and any special or general rules of the road. I just consider it part of my normal research for any trip.
Our rule-of-thumb for European travel is that if we are only staying in a major city or travelling between two or three cities, then we generally travel by train. If we are on an extended visit (3 or 4 weeks) and want to visit smaller towns, more rural areas or cover a region, then we always rent a car. In the last 10 years, we have always used Hertz in Europe with great satisfaction. A car allows you to explore areas you wouldn't otherwise normally see. It is part of the adventure.
While I am certainly competent with a manual transmission, I prefer to drive an automatic in Europe, since it is one less thing to have to think about while driving. It also means my wife can take over if necessary, since she is not comfortable with a manual. In the UK and the British Caribbean, where they drive on the left, I insist on an automatic, as I have never gotten used to shifting with my left hand! We normally arrange for a class of car that is guaranteed to be automatic, such as a BMW 3-series, an Audi A4 or a Mercedes C-Class. Smaller automatics are certainly available, but we want a trunk big enough to hide our luggage when changing locations. On our last trip to the UK we got upgraded to a Mercedes E-Class, which being a much larger car was a bit nerve-wracking initially on some of the narrow Cotswald roads, but I got used to it. I just accepted it as a challenge, and ultimately enjoyed it. It was a beautiful car!!
In Italy, an international driving permit is legally required, contrary to another poster above. If you don't have one, it may not be a problem, but if you get into an incident understand that you can be fined if you don't have one. Please note that an IDP is a permit, not a licence. You still need your valid provincial or state driver's licence!! The purpose of the IDP is to certify and translate your licence into multiple languages, so that a local officer that has a poor grasp of English can understand it. Technically in France an IDP is also required but I have never bothered with one there, since my Ontario licence is printed in both English and French. In the UK, generally only your valid provincial or state driver's licence is needed.
I love driving and generally find driving in Europe to be much more enjoyable than in most areas of North America, particularly the congested eastern states and provinces. The lane discipline is so much better in Europe. Large trucks are only permitted to drive in the farthest right lane. They can move to the next lane to the left, but only if necessary to pass and then must move back over to the right-most lane. Unlike here in North America where large tractor-trailers are often spread across every lane of a multi-lane freeway. Smaller vehicles also tend to only use the left lane to pass and then pull back to the right, always yielding the lane to faster traffic. None of this sitting in the left lane self-righteously, right at the speed limit and never yielding it to other traffic, that is so common on this continent. In my experience if you follow their rules and keep right, so that you are neither a hazard nor a hindrance, you will be respected and not be bothered by either the locals or the police. And it actually makes driving on the highways so much more relaxing.
Enjoy your trip and don't be afraid to drive.
Couldn't have drove between the places we visited without GPS.
