This was exactly my experience except Masters instead of Bachelors. I had this feeling that I mostly coasted during my bachelors, only putting any effort the week before Finals or Unit Tests. I did my Masters in EE from a university renowned for being a tough program. I was up for the challenge, doing exactly what you did: thinking math all the time, feeling like I was wasting time any moment I was not in front of my books.

I have the exact same problems you mentioned: not being able to just be, always anxious to be doing something productive, can't bring myself to watch a movie unless the movie was an all-time classic and "worth wasting time on".

The pandemic, weirdly enough, brought me back down to Earth. I faced some real mental lows but now I am able to relax more. Time management and deep work a few hours a day goes a lot further than just fretting about being productive all the time. I still have a lot of work to do, and I still don't think I've fulfilled my potential but posts like yours have definitely helped me re-calibrate my expectations.

Thank you very much.