I am trying to teach my youngest son this. He is 19 and 3 hours away at a trade school for disadvantaged and disabled kids operated by the Department of Urban Forestry. He is learning disabled (he reads on a 4th grade level) and socially phobic. (his dad has him on his insurance but won't send the child to a therapist to figure out exactly what is wrong-*rick)

Anyway, he always wants to come home. Poor kid. But finishing school is in his best interests and he knows that. He shares with me how he is feeling and I remind him about his self-talk.
To calm himself I always suggest things like picturing the beach. Not all the people and activity, just the beach, the waves and the sound of the ocean. He has been many times so it is not hard to do.

And the other is to remind himself that he is not in danger. He has food, shelter and clothing. (His environment is very strictly supervised. No drugs, alcohol, fighting or pranks or permitted. You will be expelled right now). All he needs to do is seek out his bay adviser. And of course I am a phone call, text away and 3 hours physically.
It does work for him. But it is easy for panic to overrun him and so we have lots of reminders.