It's a great time of the year to be seeing Europe, let me say. Fall is my favorite.
There are different types of travelers, and LittleMiss speaks for many who prefer the freedom of winging it and going with the flow. Unfortunately, having very limited vacations and time to do my travel, I have to be a planner, like it seems you are. I wish I could just raise my sail and let my experiences blow me where they would, but I have to decide what I want to see, prioritize, and so on. I don't want to waste 2 of my 5 days, for example, checking out a place that I end up not liking, while missing one nearby that sounded great.
That said, I think the best thing about your itinerary is you give yourself a decent amount of time in each place. My minimum satisfactory stay is usually three days...if I'm zipping around more than that, I don't feel like I've really connected with a place. In that vein, I'd likely nuke the one day in Florence and give to Barcelona, or Venice. I spent one night in Florence and have virtually no memories of it. Go back when you have more time to spend there.
One thing you might want to look at is to pick a place or two that isn't a major, European capital or city. Find a small town that sounds idyllic and has some sights and hiking and atmosphere that sounds like you could get lost there for a few days.
But remember, it is YOUR trip. You know what gets you excited as you read about it, you know what you've always wanted to see, so just follow through on those dreams and you'll be fine...
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"Life is not measured in the number of breaths we take, but by the places that take our breath away..."