How do I cope with always having to perform calm at work?

You can hold a cabin together through anything. And part of the job you rarely say out loud is the performance: staying smooth and reassuring on the outside while you manage whatever is really happening. That performance is a genuine skill, and it is also genuinely tiring. Being tired of it does not make you worse at it. Understand what the calm is for. Your steadiness is the thing that keeps a hundred strangers from panicking; it is a gift you give them, not a lie you tell. But a mask worn constantly with no place to take it off starts to wear the person underneath. The calm is for them. You need somewhere it can come off for you. Make space to stop performing. After the hard flights, find one place or one person where you do not have to hold the face, where you can be tired and unpolished and honest. The calm you give passengers is real work. You deserve somewhere to set it down.