With such a long trip, you have the time to slow down and relax. You can still never see everything, but you will come back in the future!
Try to stay a minimum of two nights in one place, otherwise you ate always on the move. And every couple of weeks, you need to stay somewhere for longer: 4 or 5 nights, just so you can relax, and get to know a place.
It’s also nice to base yourself on one city, and do day trips from there to visit other places much more relaxing that way.
There are some suggested itineraries on the Eurail website.
https://www.eurail.com/en/plan-your-trip/trip-ideas/suggested-itineraries
