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!
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.
Your anxiety is a product of society. Society is supposed to serve human nature. We have it the other way around (human nature being forced into unnatural feeling situation to serve society).
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.