I like doing a loop like yours.
I would definitely stop at Prague when you leave Berlin, and you could stop at Venice and/or Florence before heading to Rome. There are a couple nice day trips out of Rome as well.
I am curious about the Rome/Barcelona leg of your trip. You plan on the train? Perhaps you could fly Vueling or some other inexpensive airline, or take the ferry? The ferry would probably take 24 hours or so though.

I did not list the places I want to go to in order. I am saking for help for that… Rome is where I’m from so I just want to go visit family. My stop in Italy is to visit friends and family, more than anything.

With the cities you have now I’d do London, Paris, Amsterdam, Berlin, fly to either Rome or Barcelona and then fly to the other one, but this could change quite a bit depending on what cities you add and what plane ticket you buy from/back to L.A. Are you interested in art, food, the outdoors? This will help people make recommendations. Would you want to see some smaller cities & towns, or stick to major cities? Many here will recommend throwing in some smaller towns, which is fine, but I see nothing wrong with all big cities if that’s your thing.