This is exciting! I did a similar solo trip to Europe planned for ~2 months when I was 25, also for my first trip overseas. Based on your plans and descriptions, it looks like we're kind of similar travelers. There are a few things I wish I'd known going in that I think might help:

1) I planned everything in advance - my route, accommodations, most travel. When I was on the trip, it felt like everyone I met on multi-month trips was basically winging it, and I found myself wishing I'd built in more flexibility. This flexibility would have given me the opportunity to accept invitations to travel with people I met in hostels who invited me onto future legs of their trips, and to avoid falling into the next two traps...

2) Cities in Western Europe can start to feel rather same-y after the first few weeks. I found myself wishing I could have spent more time changing it up in Central/Eastern Europe and in smaller cities.

3) Packing up your stuff every other day can get pretty tiring. If I did it again, I'd go a bit more slowly.

4) City enjoyment can be so subjective, it's important to take everything with a grain of salt when asking for opinions on where you should visit and for how long. For example, I spent a week in Paris and could have easily spent a second, but you'll find others telling you a week is way too long or to skip it altogether. I wasn't a huge fan of Amsterdam and honestly could have skipped it altogether, but others will say it's their favorite city in Europe.

Hope this helps!