You're planning to go to northern cities in mid-winter. That means a lot of heavy clothing to tote around. And in those northern cities you won't have a lot of daylight. Sunset in A'dam at 5 pm, Berlin more like 4.30. Expect gray skies, not sunshine, and be pleasantly surprised if the weather's nice. I wouldn't count too much on theatre or comedy, unless you are fluent in the local language. You may be able to get cheap seats for concerts (classical music) and operas. You'll have lots of history in every city, but museums can be expensive. Check into multi-sight passes which can sometimes save you money.
Bratislava is an easy bus or train ride from Vienna. Consider day-tripping instead of changing places. Which reminds me - hostels. There are a couple of websites I've used, hostelz.com and hostelworld.com, to find hostels and get lots of info about them. One thing to consider is that some "close" for several hours from mid-morning to mid-afternoon. You may not be able to arrive or depart during that time, which means you have to find a solution for your luggage. Expect storage to cost you around €5 each time.
One day in Venice isn't worth it. Plan to spend at least two full days. Consider splurging on a one-day vaporetto pass, ride along the Grand Canal early in the morning (even pre-dawn), take a ride to the islands - worth it even just for the ride across the lagoon - and then the Grand Canal late at night. Other than that, Venice is best on foot. It's no more than an hour from end to end . . . well, more if you get lost, but that's part of the joy of being in Venice.
I wouldn't go to Sicily for a week. It needs more time, even with a rental car.
Lastly, I agree you have too many locations. Use bahn.com for train schedules - find out times and durations of your proposed train rides. Remember that you have to add time to get to/from train stations. Also, in each new location you need some time to get oriented/adjusted - where your hostel is and where the sights are, how to use local buses, trams, metros, just finding street signs can be a challenge, signage in different languages (like entrance/exit signs), and in some places getting and using local currency. Get a copy of Rick Steves's Europe Through The Back Door - lots of very useful information and tips.
Lastly, I agree you have too many locations. Besides the travel time from place to place, you will need to do laundry and shop for things as you run out of them. There will be snags - there are always unexpected things that happen - or don't happen. Having fewer stops gives you more lead time for them. Avoid the kaleidoscope effect - seeing too many places too quickly, they can get all mixed up in your memory until almost nothing stands out. Less is more. Giving yourself time to enjoy every place you visit is better than trying to see every place.