I took my first trip to Italy (solo, too) last October. I chose a shorter trip, so I focused on the north. First and last nights, Milan, 4 in Venice, 5 in Florence, 1 in Cinque Terre. I found that the one night stays didn't involve packing/unpacking. Even so, I was pretty worn out by the end (and I didn't have long-haul flights or jetlag to deal with).
I did all my travel by rail. There were several reasons I didn't want to rent a car: [1] it's expensive when you are solo, [2] driving in Italy is a challenge and solo you have to deal with both the road conditions and the navigating, [3] some of the cities (Florence, Siena among them) have dreaded no-drive zones (you can read more on this forum), and finally [4] the trains are good in Italy.
So I limited my itinerary to the cities with good train access. I managed a full day in Verona and 1/2 days in Padua and Pisa while "training" from city to city, by checking my bags at the train stations. Even so, changing locations eats up a half day, what with packing, checking in/out, getting to/from train station.
Venice - I fell in love with it (and can't wait to go back - Feb 4th). I had 3 days, one of which I "wasted" on a trip to the islands (Torcello and Burano). Much of Venice is not touristy and most of the tourists are 9-5 day-trippers from the cruise ships. I stayed in the Dorsoduro section, which is very close to the "action" but has a definitely quiet and residential atmosphere.
Florence - If you love 14th-18th century art, and especially sculpture, 2 days in Florence is barely scratching the surface - and there is more. I had 4 days, one of which was a day trip to Siena, and didn't get to everything on my must-see list (while I loved the sculpture I did pretty much skim through the other art).
If I were planning your trip, I would drop Bologna and add that time to Venice. I would see Siena as a day trip from Florence and add those nights to your Florence stay.