Anxiety is a form of arousal mixed with fear. A panic attack is when the arousal associated with the fear becomes too high and overwhelms the person.

You can try to change your thought process so that you convert the arousal associated with fear to the arousal associated with some kind of positive expectation (excitement). If you're anxious, it's because you're aroused by what could go wrong. If you're excited, it's because you're aroused by what could go right.

If you monitor the thoughts of your inner voice, while you're anxious these will revolve around what can go wrong. If you intervene at these moments you can replace those thoughts with excited thoughts. So to use poker as an example, if a hand is underway and you're thinking to yourself 'fuck i could lose my stack here if i mess up, then how will i pay my rent and buy my girlfriend that ring she wants, and zomg fucking hell zomg', the instant you notice that thought happening you can consciously intervene and tell yourself 'heyehey i could win a big pile of money here, move into a nicer place on the beach, and get my girlfriend a nice ring'. Just saying the latter words, even if you don't necessarily believe them, will help convert your anxiety to excitement. If you do that kind of intervention regularly enough soon the internal voice will start saying the positive words as a matter of habit and you won't need to do it consciously anymore. Because the unconscious is basically stupid, just hearing those words in your inner voice will change your emotional state.

Of course that doesn't guarantee you will win more money, because that depends on how you play. But you will experience a more positive feeling while you do play.