Here’s the difference between mentally preparing and worrying:
Mental preparation is decisive and active. You think about options or situations ahead of time and decide how you will handle them. You resolve something here.
Worrying is indecisive and passive. You also think about options or situations when you worry. But you don’t make a decision on how to handle them, and then the situations happen to you. Worrying does nothing to resolve a concern or problem.
Even if your preparation turns out to be incorrect, at least you will not have wasted time as much time worrying.