I understand the feeling of getting panicky in the heat. Have had it myself in the past. Possibly because you associate it with having a fever and being ill, which is a bit scary. Anxiety can make your temperature rise, so it’s possibly a bit of a vicious circle. Other than of course staying cool from a temperature point of view (cooling fans, cool water etc) I think that it’s unfortunately a mind over matter thing (not overly helpful I know) and when you find yourself worrying about it, try to switch your attention to something else (reading a book that interests you, watching a film/TV show, call a friend and have a catch up, any hobby you have- basically anything you like doing). The calmer your mind, the less you’ll focus on the heat issue-I think it’s a lot to do with distraction. Hope you can find a technique that works for you, but certainly understand your concern around this issue, as have definitely experienced it myself too in the past.