While it seems reasonable that you should try to relax, the trouble is that the severe anxiety has too strong a hold and you cannot simply choose to relax when you are being thrashed about by your own anxiety. Yet you can stretch and tense your major muscle groups, and you ca breathe deeply. The stretch and tense combination consumes the physiological aspects of high anxiety, and as you let go, your muscles naturally relax. You should continue the sequences until your panic collapses in on itself.
The Don't Panic Training involves multiple stretch-tense-air-release/relax-suggestion sequences, in several variations. The sequences stand up to the power of high anxiety, and reduce anxiety for 70% to 80% of participants. Practice the training several times, and you will significantly reduce your excessive anxiety.
The Don't Panic Training is also an excellent solution for those who lie awake and are too worried to get to sleep. The training reduces your worries, for the moment, and makes it easier to drift off to sleep.