Fun stuff! Mental panic for no good reason (not meaning to sound harsh so bear with me) is a trainable thing. I've done any number of dangerous hobbies and used to have a dangerous job. Every one of them has come with a unique type of OMG moments that I had to train myself out of. 90% of it is trusting your gear and that trust only comes with repeated use.
I'm not a claustrophobic person, as long as I can breath I'm good for the most part. But suit up in full fireman SCBA, turn off the lights and crawl through a tangle box and bam, panic starts setting in. I can't move, I can't see, I can't (I can) but my brain says I can't breath and I can't think. And you're in the exact same mode with the 1st post. Your mask is flooded, you think you can't see, you think you can't breath, panic. So your first step is a simple one, that will fix a complex one. Back to me
So I'm in a full dark room, massively tangled up in wires, heart rate is shooting up, I REALLY want to pull my mask off because it's not keeping up with my skyrocketing pulse and breathing rate, but I can't, because in the real world that = death. So I'm only the 1 billionth person the person training me has seen (he's got some night vision on) go through the exact same panic and he tells me how to fix it. "Close your eyes."
That's it, magic bullet. It sounds stupid, it's completely dark and I can't see, so how is closing my eyes going to do squat? But I listen, I close my eyes, I take a breath and bam. I can think. Heart rate drops, breathing drops, brain fires up, I work the problem an crawl out of the box. When your eyes are open but you can't see, your brain panics. Panic brings all those problems. But, close your eyes. You STILL can't see, but your brain says "Oh, eyes are closed, perfectly normal." Your brain works again, you can tell yourself I CAN BREATH because you can, and you work the problem. Simple step, complex fix, life is good. The same works under water.
Now here's the thing. Your mask is going to get water in it from time to time. It's possible you may lose your mask. And here's the 2nd thing. IT DOESN'T MATTER. As long as the reg is in your mouth, you're 100% fine. Close your eyes and tell yourself "I can breath" Take a few breathes, relax. Let it flood, it's annoying, nothing else. My eyes are tough, I spent years in pools with varying amounts of chlorine, I'm used to irritated eyes. I can (don't like to) swim in the ocean with no mask. That's all that saltwater is going to do. It's going to sting, it's going to irritate your eyes and that's all. When I did my drills to get cert I was one of the few that pulled/flooded with my eyes open. It hurt, but no big deal. Mask on, clear it, done. Anyone could have done the same, just most people worry more about stinging eyes than I do. And to be honest salt water sucks less when your mask is flooded than when it's just splashing.
Anyway, long story short, my advice would be to gear up and hop in the pool. Spend some time flooding, clearing, LETTING it happen in a controlled environment until your brain finally figures out IT DOESN'T MATTER. After that buddy up and hit the ocean. Do it again with someone right on your hip. Flood, clear etc, do it with your eyes closed until your brain knocks it off. Once that happens, you'll be 'cured' for life. And if in the beginning your panicking CLOSE YOUR EYES and do everything with your eyes closed. It works. It's stupid, but it works.
It's far less panicking than how I used to teach rock climbing. I made you climb until you took a few good falls. I *KNOW* I can drag a semi up a cliff with this rope. But until YOU'RE convinced, you're not going to trust the gear and you're going to freak out because you might fall. Trust me. You're GOING TO FALL, it's no big deal. You fall, you climb again. Ez peazy