Also, who says I can't control it? Maybe if I keep worrying about it, I might find a solution! It's happened before!
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
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.
Third reason - you shouldn't worry but you still do...
The thing about it is, you can ALWAYS control what you want to, you'l just be punished if you do.
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.
Also, who says I can't control it? Maybe if I keep worrying about it, I might find a solution! It's happened before!