Those who do not suffer from anxiety do not always understand the person who does. That being said, while communicating with them, they may inadvertently say phrases that make such people feel unsupported and even more irritated. Note that he never mentions the two phrases below.
1. "Calm down"
This suggestion sounds more like an insult to someone dealing with anxiety, despite your good intentions. If you can't talk to or deal with someone suffering from anxiety and you give this answer, you're just giving them a "I can't deal with you" message.
Alternative:
First try to understand what they are feeling and if you can't think of anything else to say to them, ideally it would be simply: "How can I help you?" Make sure that your tone of voice is empathetic and let them know that you know what they are going through is not easy at all.
2. "Just Don't Think About It"
Anxiety itself is constant tormenting thoughts, so this is the worst phrase you can say to someone. Because thoughts are precisely the peak of his suffering. It is much harder than you think to ignore a thought that is constantly there, tormenting you mentally and psychologically.
This expression will only irritate them and make it worse because if it was that simple, they would have done it.
Alternative:
Rather, create a dialogue about the issue that concerns them, listen and try to support them by giving them a constructive choice. So quote what is causing him anxiety and get him to think about something other than what is currently troubling the anxious person. Always being careful.