Expressing that you are prepared because you feared the worst

I was prepared for the worst case scenario.

A worst-case scenario is a concept in risk management wherein the planner, in planning for potential disasters, considers the most severe possible outcome that can reasonably be projected to occur in a given situation.

Wikipedia

Government planning for ‘worst-case scenario’ on extradition with UK

Irish Times - 8 June 2018

Training for the worst case scenario

Springfield Times - 6 June 2018

Hope for the best and prepare for the worst. (and Hope for the best but expect the worst) TFD

Proverb. You should have a positive attitude, but make sure you are ready for disaster.