I did my OW training just prior to my 58th birthday. I was never scared during OW or AOW training as I had done some Discover Scuba dives over the previous 12 years so was very comfortable during training. But for whatever reason, and I'm not really sure why, since then I have had a couple of anxious moments while diving, and generally it has been while just beginning descent on ocean dives. I suppose it was that the viz was somewhat low and I couldn't see the bottom when submerging even though I had an anchor line to go down. But I was able to quickly get my senses about me, get over the anxiety, and continue with the dive without incident. Now, instead of just dumping my air and going under, I will stick my face into the water for a few seconds, look around, get used to breathing, and then dump my air. It's usually so quick nobody notices I'm doing it. And it's only on the first dive of the day. After that, i'm fine on subsequent dives.
One incident, tho, was worse than others. Me, my wife, and another lady were diving together in a local quarry. We had been in the water only about 6 minutes and were following lines at 30 ft to a sunken sailboat at the far end of the quarry. There are many tees off the line to different things in the water and I realized I had missed the tee I needed to follow. Just as I turned around to go back, I started hyperventilating. My only thought was to get to the surface, which I did. I don't know why I started hyperventilating; I wasn't scared or uncomfortable, it just happened. We decided to not continue for the day, and for the next few dives, I had that incident in the back of my mind and had to work my way thru anxiousness for a few dives, especially around the 6 minute mark of the first dive of the day. It was just a mental thing I had to work thru.