I feel your pain (as will most of us here). My two tips are:
1. Tell yourself to learn just one thing that you didn’t know before. Pick a problem to work, and learn it flawlessly. There is elegant beauty in math and physics; just let yourself enjoy it and do it for the sake of learning whenever you can. (If you can do this, sometimes you’ll start early, which takes the last minute pressure off).
2. One tip from another poster on this forum was brilliant. Ask yourself what is one thing that you can do to get closer to your goal today? For example, go to your study, then open the book, then pick another small thing that will get you closer to your goal. Write down the steps beforehand if that helps.
Again, I know what you’re referring to, and it tormented me all the way through my dissertation defense and well into my first year as an Assistant Prof. I think you’re wise to try to deal with the issues now.
Good luck. I believe in you!