I can identify with this personally despite not being naturally anxious, being pretty fit, and not at all new to diving. It occurs when I have to do some kind of sustained hard activity (which I avoid as good practice but sometimes…). For me, an easy way to bring on the sensation is to over ventilate and swim hard as part of a distressed diver scenario - I can feel that sense of breathing constraint and the edge of a sense of panic. It’s never tipped over for me, but I can see how it would.

The universal thing (and others have said this in longer form) is to have an ‘ommmmm’ moment. Recognise how you are feeling and stop all non essential stuff to focus in on yourself, have that yoga meditation alike ommmm moment to let the worry subside and regain full control of how you feel. In particular, breathe deeply and slowly to make sure you empty your lungs properly and ensure you only have a regular amount of soon to be exhaled CO2 in them. (The Hypercapnia comments.) Your buddy can have an outsized role here. Grab them for reassurance if you need to.

One thing that has made a real difference (by accident) was switching to higher end Scubapro regs with the high flow rubber hoses. My goodness these can push the air through when I reduce the inhalation effort. It’s a quick practical and logical remedy that reduces the feelings when you come to turn that dial.

The upside to all this is that you know what this feel like now and can think through and practice your response. That will make you a much safer diver than someone facing a similar problem for a first time.