Well I don't think it helps to name/judge it with "catastrophizing" and you aren't. You are overlaying a previous reality onto the future that hasn't happened yet; it is pure fantasy and speculation and guessing and that's scaring you, where I think that the reality is that you will be able to handle this clerk and her attitude, which is about her (her inability to understand, relate, be compassionate) and not about you. You don't even need to acknowledge her attitude; you are an innocent bystander of her grumpiness.

This would be something that could be beneficial to you to talk about with your therapist.