I so understand what you mean. People say “just don’t worry, because it won’t do any good” and you want to slap them and say, "I know it won’t do any good and I know it’s pointless, but that’s like telling an amputee to just grow another limb. You worry about (some) things the way that you breathe - you just do.
One thing I find helps a lot is exercise. I really need cardio, but I know some people are more into yoga or whatever. It gives me pure physical input to focus on, and once I get going that can shove everything else out: I’m not thinking about anything but my feet hitting the treadmill, or keeping the right cadence on the bike pedals, and the bad air leaving my lungs to make room for the good air. (Also: loud music with a good beat, which more literally drowns anything out.) If nothing else, it’ll help you fall asleep at night.
Another thing that helps: avoid the topic. You know perfectly well that reading up about it is going to do a big heap of no good for you right now. If you have a question, do not Google it because then you will get lost and spent an hour reading about whatever’s worrying you - call your doctor instead. Ask. Get it over with. All reading about it will do is keep it at the front of your mind, which makes it easier to stress about it.
Keep yourself busy. Seriously, now would be a great time to take up rock climbing. Or jigsaw puzzles. Or knitting. Or cooking. Or anything that will occupy your mind. If you’re thinking about how to get up that wall or where that blue piece goes or whatever, you’re not thinking, or at least thinking less, about whatever you’re worrying about.
And finally, do you have a really trusted friend? Someone you think you can tell anything to and they might crack a joke but they won’t put you down and they’ll still be there for you? Tell them. Sit down with them somewhere private, ask them to just listen, and spill all your worries. Even if they’re totally insane - those ridiculous thoughts along the lines of “Oh god, x leads to y which leads to z which leads to me being chased around by a vengeful dead philosopher who’s been reincarnated as a snail*”? Let your imagination go crazy with how bad it could be. Get it off your chest. At worst, someone you trust knows exactly how you’re feeling. At best, you actually say it out loud and realize that it might, in fact, be a bit silly. It won’t fix anything nor will it stop you from stressing about it, but it might help for a bit.
Also keep in mind that I’m a crazy person and therefore my advice should probably be taken in conjunction with an entire salt mine.
*This might just be me, actually, but, uh, I had that nightmare once.
