Not to age myself, but my first trip to Europe 30 years ago was a whirlwind 3 week trip in a bus with 12 other people. While we saw 5 countries; Germany Switzland, Austria, Italy (Rome and Florence), and France (Riviera). What I remember today is the scenery from the bus and as we packed up to move on to the next destination, I wished I could have stayed in each place longer. Of course in those days, the Internet didn't exist and I don't remember doing any research before the trip.
In the past 10 years my husband and I have been to Europe 4 times, and the 5th is only 17 days away. So below are the countries we've visited and length of vacation. As you can see, I learned from the first trip. Don't get me wrong, my first trip to Europe was great, and was a teaser until had the time and money to return 20 years later with my husband for his first trip. But saver and enjoy. You'll go back again and again, even if it is years and years later.
2005- Portugal (8 nights)
2007- Dublin and London (9 nights)
2011- Drove from Germany (2nights) to Italy (12 nights)
2013- Italy - 14 nights
2014- UK- 14 nights
2016- Spain? or maybe France? 14 nights
One other thing I do to make sure I don't over schedule is a calculate how much time it takes to get door to door, whether traveling by car, train, or whatever. I figure I have no more than 14 waking hours that we are not in our hotel room. So if door to door transit is 4 hours (that might mean a 2 hour train ride), that leaves 9-10 hours at origin or destination to explore, chill, wander, eat, drink, tour, etc. In a 2 week trip, I plan only 1 or 2 days max where we are "traveling" all day, or more than 6 hours. So as you work through your itinerary, make sure you are not spending too much time on trains, airports, etc. Also, keep in mind that an hour flight is really a 3-4 hour door to door excursion due to security and transit to/from city centres.