How do you calm down when you're anxious over something you can't do anything about at the time?

Go back and read the last part of your question. You can’t do anything about it. What good is worrying about it going to do? Worry about things you can change. Then go change them.

Sometimes I distract myself with watching DVDs, sometimes with Fluther. Sometimes I talk to friends about my worries. Sometimes a fast walk will help sort me out.

Not that I have ever experienced such a feeling, but…

Typically 3 months after a guy has a prostatectomy for prostate cancer he will get a PSA blood test done to see if any cancer was left behind or if it has spread to bones. The test is repeated every 3 months until about a year. If you are average, during the weeks leading up the the test and the days after while waiting for the result all you can think about is the test and the result and what ifs.
Your head knows there is nothing you can do about it but that does not stop you from thinking about it when you get up in the morning, when you pee, when you get dressed, when you brush your teeth, when you work, when you…. when you go to bed and when you wake up in the middle of the night for a midnight stroll.

Or so I’ve been told.

You get on with life and pretend it does not matter. Eventually it passes. (So they say.)

Try turning your attention to what it feels like to have these thoughts. Rather than engage with the content of those thoughts, try stepping back slightly and really looking at the thoughts and seeing that they are merely thoughts. There is a difference between investigating, in a curious way, the nature and automaticity of your thoughts vs getting wrapped up in the content of those thoughts.

I find that I can go long periods lost in thought without ever realizing that I am thinking. There is instant relief to be able to just know that you are thinking – even if the content of those thoughts is disturbing. And witnessing the entire process and its effects on my emotional and physical state seems to immediately loosen the thoughts’ grip.

Sometimes distraction works, and this is all I need. But I find more often lately that distraction is a temporary solution that comes with its own problems.

Anyway, it seems that you have a good grasp on the situation. You “can’t do anything about this” right now, yet you are “consumed” by these thoughts. You might find that turning towards your experience of feeling this way (again, without entertaining the contents of these hungry consumers) provides some relief – and insight into the nature of your mind.

Apart from distractions, I try breathing meditations. I’m not good at meditation but when I’m really stressed, I find it can really help me.

This site shows you how to do a couple.

I’ve also tried using this site A friend who is battling some really severe health issues told me about it. You can use it for the first 10 days free and it takes 10 minutes. I’ve used this technique before really stressful meetings. Just to help me calm myself and regain my composure.

It usually has to do with feeling helpless. So I find a project that I have been putting off and work on it till it is done. This helps bring back the sense of control. Once you accomplish the task you may feel more in control of your life and maybe the thing you were worried about has worked its way out.

When my husband was in a terrible work situation and was applying for jobs in other places, I had him build me a large tv/bookcase. It took him two weeks. I told him to take his time and everytime he thought he was near done, I would change my mind on a detail.

By the time he was done, he had an offer to go to another branch and away from the people who were making his life difficult. And he was showing off his handy work for months to family and friends. And he slept like a baby. Instead of worrying about things he just kept thinking about how he could better improve the stand.

I will first acknowledge that I am feeling nervous and I’ll think about what, exactly it is that is making me feel that way. Then I’ll go through why I am making it out to be worse than it really is. I’ll reassure myself that it’s not that bad and that I’m blowing things way out of proportion. That usually helps me reconcile my feelings with the reality of the situation.

Anxiety is usually born from exactly that, feeling you have no control.

Do productive things you can control and distract your mind.

Plan something enjoyable like a trip somewhere or some sightseeing where you live. Having something good to look forward to usually is a mood booster. Depression usually involves a lack of hopefulness, seeing nothing bright in the future. Even if something negative might happen in the future there can also be positive things to balance it. If travel isn’t your thing then pick something else. Sign up for a class, start a new exercise routine, all those things you will have to spend a little time researching where you can do those things. Just the research will help occupy your mind. You could volunteer somewhere.

You can always come to fluther and pass some time.

Try to compartmentalize your thoughts and push the thing you are worrying about out of your mind when you start ruminating. I know it can be very hard. I wouldn’t suggest doing this if it is something that you are avoiding doing, but if it is something younjust have to wait and see, no choice, then surrender to the waiting period as well as you can and ignore it as well as you can.

Exercise, and the heart-pounding, fast-breathing kind that leaves you a sweaty mess. A good workout is an emotional and physical sponge that soaks-up uncontrollable anxiety.