One tactic that's worked for me is mapping the likely outcomes for the thing I'm worrying about. This helps me to ground my thinking and reduce the formless anxiety of the fight-or-flight response.

For example, I was recently shopping for a used car, and I was worried about what the pre-purchase inspection would turn up. I sat down and codified what outcomes were likely and what I could do in response.

-Minor repairs needed that are within my budget: Buy the car
-Major repairs needed that are within my budget: Buy the car
-Major repairs needed that are outside my budget:
How major?
-Severe issues that may cost more to fix than the cost of the car (failing transmission, steel shavings in the oil, etc.):
Don't buy the car

Of course, not every issue has variables as clear-cut as this one, but after I nailed down the most realistic possibilities and decided that only one of the four was truly ambiguous, it was very calming.