Europe 3 months

That sounds like quite a trip. Personally I'm not sure I'd be that keen on continuing after 1 or 2 months, but if that works for you, then why not? Try to make a more detailed plan and book your train tickets in advance to save a lot money. Also, make sure you do not exceed your 90 days in the Schengen area.

Thanks will do x

You can certainly spend 3 months in Europe and cover most of what you want, but... first...

Congratulations on being able to do a trip like this!

Questions #1 and #2: What are your interests? Art, Music, Culture, History....? And in each place you want to visit, which interest will be served? For example, I like the outdoors and find it reviving. I would definitely program in a week in the Italian Dolomites to hike if I was planning such a trip to use as a break in the travel. Or, program in a week at a Tuscan Agritourismo that specialized in food and wine tours for your break week. Or both.

Question #2: What is your travel style? Some folks like go, go, go. Some folks like a second cup of coffee in the morning on the balcony before embarking on the day. Build this into your trip planning.

I used to travel for several months at a time on business, visiting a few settled locations. DW and I have had one really long (35 days) European trip together. and many, many shorter ones. In the cases of the long trips fatigue became a real issue about week 3.

My major suggestion is to build in one day per week which will be devoted entirely to sitting in cafes while doing laundry or reading a book. It may seem to be "missing" things - but you will miss things anyway. And a well-rested traveler enjoys what they see much more.

So, without any other information I would suggest starting by breaking the trip down into 12 one-week/6 two-week segments. You have already identified your first 2 two-week segments - Greece and Italy. Then you have mentioned Spain - 1 week, France - 1 week, Belgium/Holland - 1 week, Germany - 2 weeks, Switzerland - 1 week, Paris - 1 week, London 1 - week, totalling 8 weeks. Trip done. You will lose one day a week for travel between destinations, but so what.

Now What do you want to do and exactly Where do you want to do it within those 1 week segments?

Have fun with the planning. Many other commenters on the forum have advice on financial arrangements for being out of the US so long, Communications strategies, travel Insurance/medications, packing, etc.

There are several things to consider when planning a long extended trip. Number one, build in some rest time every 2 weeks. I recommend 2 days every 14. I don't mean do nothing, but cut way back on sightseeing and sleep in a couple of days. You will feel refreshed to start your next 14 day excursion. I try to find an interesting location on a lake, the sea or some scenic location to just veg and enjoy a slower pace.

You are going to have a little over 12 weeks. Decide how many weeks you want to spend in each country. Keep in mind every flight will use up a day and each city/hotel move is at least a half day. You have listed Spain, Greece, Portugal, Italy, France, Netherlands, Belgium, Switzerland, England and Germany. That is 10 countries in less than 12 weeks (remember travel days).

I think this might be too ambitious for first time travelers to Europe. I would narrow it down to 5 countries and begin detail planning of what cities you want to see in each country. Try to keep those 5 countries within proximity to one another. Your wish list has you stretched from Greece to Amsterdam. You will more than likely find 5 countries will be plenty.

You may want to read this page on the Rick Steves website.

https://www.ricksteves.com/travel-tips/trip-planning/itinerary-tips

....and watch the video

https://www.ricksteves.com/watch-read-listen/video/travel-talks/trip-planning

I spend my summers in Europe now that I'm retired, and I think you are trying to do too much over too large a territory. Without knowing anything about you special interests, I'd suggest trimming at the margins. You've allotted a fair amount of time to Greece, so I assume it's a high priority. Therefore, I'd chop Spain and Portugal to allow the itinerary to breathe.

I have a feeling you haven't dug into guidebooks to most of those countries. You're going to be hopping all over the place, passing (or flying over) really great destinations to see yet another capital city with outposts of Starbucks, KFC, etc. I know the big cities are where a lot of the best museums, etc., are, but the atmosphere can be hectic. I think you'll have a nicer, more relaxing trip if you include some time in smaller cities as well. After Greece and Italy you have listed only major metro areas aside from Lucerne.

I like to mix in some smaller places. Some I get to on day-trips, but that can mean budgeting extra big-city nights and paying big-city hotel rates while visiting the usually-cheaper small towns. So I prefer to have some stays in smaller places. I look for spots where I can identify a cluster of small towns or beauty spots that can be visited from one base.

Remember that a rapid pace may be sustainable for 2 weeks or so, but you won't be able to keep it up for 3 months. I don't build in designated rest days, but I do set a pace that isn't rushed, carefully considering how much time I will need at each stop to cover my high-priority sights in a leisurely manner.

A few specific comments about your plan:

Greece: Four nights on the extremely touristy island of Mykonos? Are you sure?

Italy: Two weeks would give you decent time for the magnificent Venice-Florence-Rome but not really enough for much else, which (depending on which month you hit Italy) might include the Dolomites, the Lakes, Tuscany/Umbria, the Amalfi Coast, etc. You could easily spend your entire trip in Italy. Try to find time for at least one or two smaller towns less overwhelmed by tourism.

Spain: Both Barcelona and Madrid have multiple great side-trips. I hope you'll allow time to see at least Girona and Toledo. Most visitors to Spain want to visit Andalucía if they can avoid being there during the hot months. A rather rushed visit to the three major cities in Andalucía alone takes about 1 week.

Portugal: Portugal has many interesting smaller towns. I wouldn't go all the way to Portugal just to visit Lisbon (which I do like). It will add a couple of ugly travel days (or nights--the only train from Madrid to Lisbon runs overnight).

France: Bordeaux is a handsome city but not a place I'd include on a quick visit to France unless it's a stop on the way to the Dordogne/Lot. It's a long slog from your other destinations. Four or more nights in the Dordogne would be a nice change from the rest of your trip, though I don't know what the weather's like in the early autumn. If Nice is on the list to get you to Provence, you need to adjust your plan; it's too far from the classic Provencal destinations. Nice is a convenient base for seeing the Riviera towns. Why are you routing yourself through Paris twice?

Belgium: Most people would suggest somewhere other than Brussels for your Belgian time.

Germany: Berlin is a sight-rich, sprawling city. Check out the sightseeing options carefully before deciding how long to stay. Munich is generally considered to need less time, but you must allow for the Oktoberfest.

Switzerland: Lucerne is a pretty town, but most people making a first trip to Switzerland prioritize the Alps. However, I don't know anything about the weather there in the autumn. It can be overcast and rainy even in the middle of summer.

You can use the Deutsche Bahn website to check travel times by train.

Weather-wise, I would reverse the order, starting in London and finishing with Sicily and Greece when it is not so hot. Munich Oktoberfest starts in late September so you can plan around that.

This is how I travel. I do three of these a year.

You have been given good advice to plan rest days. Unless you are twenty something you can't keep up a pace of a few days here, a few days there without a break now and then.

This is your first time to Europe and I understand the desire to see everything. But it is too much. Seriously go through your itinerary and cut out things that you can see on another visit. You will be in culture shock since you are not used to the way things are done in any of the countries you want to visit and that causes extra stress.

Don't plan down to the minute. Leave some extra time. You never know what you might find out about while traveling or things will happen that will cause you to rearrange your plans (strikes, weather, etc.)

The time you are planning to travel is the busiest travel time of the year. Everywhere will be crowded. Give yourself extra time to see things as you might have to wait on lines.

Since I see this post has gone silent, I'll venture a reply. See my profile for details of my first and only 54-night trip, which somehow might help you. I don't necessarily agree that everyone will be tired of traveling after 2 months, so you have to be your own judge. My success in keeping excited during a long trip was to limit the number of hours per day I was on the go to about 9 hours or so. The rest of the time was down time, doing laundry, taking care of self, or maybe just wandering the streets if bored. I also used long train rides, such as the 6 hours or so from Munich to Berlin, to refresh myself and pay attention to thoughts and feelings. Also I usually just had one agenda item per half day, with both halves sometimes assigned to one location, such as the British Museum. If you schedule something for every 2 hours, I would imagine you would be worn out.

In terms of itinerary, if I could talk you out of having to be in Munich during Oktoberfest, you would have a smoother flow. There's plenty of drinking and general merriment possible in Munich every day of the week. If you would come around to that way of thinking, you could do:
-Greece as you planned
-fly to Naples, then trains to Rome, Florence, Venice, and then Milan in 14 days
-train to Nice
-train to Barcelona (long ride, but I would do fine with it--you may be different)
-train to Madrid
-bus or fly to Lisbon
Here I would urge you to purge Bordeaux, which is minor compared to other places on your list. In any event, I would change the order of the next part. Suggestion:
-fly from Lisbon to Berlin
-train to Munich (yes, I know, not "beer feast" but still beer flows freely)
-train to Zurich, but immediately go on to nearby Lucerne for the day if you like, but you should continue on to stay right in the Alps, right now I'm thinking Lauterbrunnen
-train to Paris, possibly via Geneva, allowing you to see more Switzerland on the way
-train to Brussels for a little time in Belgium
-train to Amsterdam
-train to London

The next step could be checking out the transportation time in detail, and then planning a tentative on how many days in each place based on your half-day lists of what you want to see. This is still a long list of destinations, maybe 18 in 90 days, so when you make the list of sites, you might end up cutting destinations. Or if it starts to feel exhausting finding hotels, learning about all the sites, and figuring out transportation, it probably will be exhausting to you on an actual trip. Hope this helps a bit.

Do it going the other direction. Fly into London in July and fly home from Athens in October. Greece is miserable in the summer - crazy hot and crazy crowded. But in October it will be much better.