Hi Tess,
Considering that catastrophizing is a type of automatic negative thinking, I believe the following would still apply:
First and foremost,try and be aware: What is going through my mind? Then proceed to asking yourself, "Is this thought helping me?". Chances are it is not. There are a series of questions you can ask yourself that can help you challenge these anxious thoughts. Also, I think rationalization is key to challenging a lot of these types of thoughts. Look for evidence, for example: "Whats the evidence for and against my thought"? or "Am I jumping to conclusions without looking at all the facts"? Consider searching for alternative explanations. "Are there any other possible explanations?" or "Am I being too inflexible in my thinking?". Then begin to put thoughts into perspective: "How likely is it that the worst will happen?" etc. Then ask yourself what would be a more helpful thought to be thinking instead...Hope this helps you a bit. And remember, practise makes perfect.:)