1. Take it gradually. Perhaps first jot down some thoughts in a notes app or on paper. Also, think about your reply in terms of drafts - you don't have to send the 1st draft anywhere. Write the 1st draft down somewhere just to get the main ideas out of your head, and then perhaps do a round or 2 of revision. But the 1st draft is just for you, to clarify your thoughts.

2. Take a walk to figure out what you want to say. 20 min walk in nature (or a park) boosts concentration, and helps you figure out what you want to say. A walk before a difficult task clarifies your thinking, so that when you sit down, you know what to do or say.

Hope that helps.