Catastrophizing is predicting or assuming the worst case scenario is the most likely outcome or the only outcome for a situation that is probably only moderately or mildly problematic. It's a form of Negative thinking that, as the name implies, involves extremely distressing thoughts
"If I'm late to work, I will get fired, then never find another job, then go broke, then lose my house, then get divorced, then probably end up on the street or in prison.”
“My Doctor wants me to call him, I must have cancer.”
Though bad things do happen in life, we need not live as if they are always going to happen. In fact, worst case scenarios are typically statistical anomalies. Most would agree a plane crash would be a catastrophe, however, their frequency is so slim that travel on airplanes is considered one of the safest available. See the statistics here.
How do I Stop Catastrophizing?
Thought diffusion, and Thought Replacement is good techniques for de-catastrophizing and for challenging negative thoughts.
Catastrophizing is also associated with a lot of Panic Symptoms. Deep Breathing is very good for reducing anxiety and panic as is Progressive Muscle Relaxation (PMR).