I use a lot of guide books, while they are quite similar, there are subtle differences which I can detect. Each traveler has different partiality and writes effusively about certain things and you can pick up on them. Others just wants to visit and get the job done. So with a collection of guide books I can pretty much piece together a few places that would most excite me and then string them up the best I could and form an itinerary. That is basically it. What places would you most want to see? One has to understand that one can't see everything in one itinerary, so it is necessary to prioritize. Create a smaller area to cover that can be easily linked by trains or buses. A bigger area might warrant flying, like an itinerary that includes Paris, Lourdes and Rome. When using the trains or buses try to shorten the distances between cities by finding interesting stops eg. Venice to Rome, try to make stops in Bologna and/or Florence for a few days. Even in a particular city, take day trips to places of interests nearby eg. day trip to Fiesole or Siena or Ravenna from Florence.
Try to make more trips, more often and thereby travel more. It would mean working more and saving more. I'm always looking for ideas to save more money. There was a time when I could work more. Keep the old car for a little longer or for a few more years, drive it less, walk or take the buses. To afford travel these days, one must make a lot more sacrifices. I remember when US$1 was equal to 1.2 Euros,that was 7 years ago, since then things has gotten more and more expensive each year and the dollar has fallen against every currency in the world. There's just no way that we can live the way we did even 6 months' ago.
Try not to double back, fly in and out of the same city, do a loop. We did that once, flew into and out of Paris. When we flew into Paris, we took the TGV at Charles de Gaulle airport and headed for Tours to see the romantic castles of the Loire. Then headed south to the walled city of Carcassone, then to Avignon and while in Avignon to Isle sur Sorgue as a day trip and then back to Paris. The other thing is to fly 'open jaw', fly into Naples, Italy and leave from Nice, France.
I've never followed suggested itineraries in guide books. I always make up my own and they are weird because my interests are so different from everybody Else's. Yours would, too, because you are different, try it, you'll find out who you are. I have changed itineraries many times in the planning stage, another thing about independent travel, you have that luxury. Not so with a cruise or a guided tour, someone makes the itinerary and you just tag along.
I am looking at my note pad to see all the itineraries I've made up for future trips, one day I may decide Malaysia is a priority, the next day I'll change my mind and head back to Europe, but eventually before the trip I would have pinned myself down to something and make plans for that final itinerary. Oh, the glorious struggle.