First you need to remember that there is only one emergency. And that is ??? Right out of air. Every thing else is just an annoyance. Easier said than done to put in practice, but you have to believe that. When the mask comes tell your self I have air and things will be OK then breath slowly out your nose to keep it water free. That should stop the flooding and drowning sensations from taking over,,,,, close your eyes. , If you cant see then you done’t need them feeling the water also and fueling the survival reflex.
Take your mask in the shower, fill it with water and put it on. work on getting over the feeling of water in the nose and initiating the cough and sneezing to clear the nose. I trust you can swim in a pool and it is the transition of mask to no mask that is getting to you.