MFAer and one time Euro-traveling backpacker here. Here's what I've got for you.

When you're traveling through Europe, you want to be as stylish as possible. When you're backpacking, you want to be as comfortable as possible. Where you want to find yourself is at a perfect middle ground. OCDBs are solid, as always, but I'd get some basic t's of various colors. They're more comfortable with the pack, and at some point, it all becomes about what's comfortable with the pack.

You want a coat or jacket. You're going to experience rain in most places right now. Plus, you never know if you're going to improvise your sleeping arrangements (we ended up sleeping in a park one night, literally in a bush). Something down would be best so you can stuff it small and fit it in with all of your other clothes.

Don't pack too much pant wise. A pair of jeans or chinos lasts you more than one wear and you want to be able to fit other things in.

As far as underwear goes, I'm not entirely sure on your arrangements, but you have to consider that washing machines are sometimes few and far between. Get underwear that wicks (something meant for runners) or underwear that's cheap and disposable so you can buy new pairs while you're over there. It's a gross practice, but back packing is sometimes a trip that requires gross practices.

If I were you, I'd leave a decent amount of room for stuff you're bringing back. Euro fashion is awesome, and you can get some interesting stuff you won't see in the states.

Hopefully, I didn't make it sound too nasty. Backpacking is life changing. Going bare-bones is a great way to do it because it will make you really appreciate your wardrobe when you get back, but you'll know that you could live simply if need be. Have an awesome time!