You shouldn't worry about things you can't control

Joking aside, this advice comes from epictetus Manual (also known as ‘enichridon’ or ‘handbook’).
It’s very short, effective and (contrary to a lot of latter philosophers) very easy to read and understand.
It helped me a lot fight my anxiety, you should be able to find it for free on the internets.
Worth reading, in my opinion.

I have not been able to find a copy yet, which fills me with anxiety

https://classics.mit.edu/Epictetus/epicench.html

There, there. Everything is going to be the same but you’ll feel a bit less anxious about it.
It will help you focus and chose to act where you actually can make a difference.

I didnt read the whole thing, but it seems similar to concepts in Zen Buddhism and seems to boil down to:

• align what you care about to things you can directly control
• spend as little time/effort possible on things you can’t control
• never attribute to yourself things you didn’t cause to happen
• always look for what you can control in a given situation

The examples there are great, such as seeing sickness or injury as a hinderance to your options, not your ability to choose. If you can frame things in terms of what you can and can’t control, and then make decisions on how to deal with them, you put a lot of things that would cause anxiety outside yourself and can limit focus to things you can control. For example, I can’t control whether my boss gives me a raise, but I can control how I present myself to that boss and whether to look for other job opportunities.

I’ve found that I mostly worry about the unknown, and if I can understand what those unknowns are, I can replace the anxiety with dread.