You have to keep reminding yourself you're going into an alien environment. And the gear you employ to take you there safely is cumbersome and uncomfortable. It's pretty natural to start second guessing yourself. Think of it this way. If diving was really easy, wouldn't everyone be doing it?
But they aren't, and most will tell you it's because they'd love to, but they're afraid ... of the water, of claustrophobia, of boats, of the crazy gear, of failing, whatever. It's something lots of folks wish they could at least try, but something stops them, usually before they start. So why expect it to be a walk in the park?
I felt very nervous when I first tried it, and the conditions were ideal. I wanted to bail but pride kept me going, and curiosity. The first dive was something like 25', and I was impressed with me that I survived without quitting, bolting or freaking. Then I screwed up my courage and dived to 65' on a warm sunny reef. That took some mental arm-twisting too, but then after I'd done it, no problem. Then I went to 85' in a cold dark lake instead of nice warm sunny clear water. That was a leap of faith, but great once I got to the wreck and started snooping, but it was damned cold. Then I went to 115' in a different cold dark lake and it wasn't traumatic at all. In fact we did that wreck 3 times in two days and I actually got bored with it. Then I twisted my own arm again and did a cavern in Mexico. I was scared spitless for the first 30 seconds going in, and then I realized that what I was experiencing was just too cool to be scared. And on it goes ...
So to me it's pretty natural to be scared and uncomfortable when doing your first dives, and then later when reaching for the different plateaus as you progress through your dive experiences. Every now and then I find myself in an uncomfortable situation during a dive and I get the doubts. Why am I doing this to myself? I really need to get my sorry butt back to the natural air and stop this stupidity. Stuff like that. But then I give my head a shake and remind myself that I really can do this. The first dives are just a test you put yourself through to see if you have what it takes. Later you will look back and laugh at yourself, but for now just keep your wits about you and be a good new diver. Overcoming the fear brings it's own kind of rewards, and the diving brings the other good stuff later.
BTW. I was 50 the year I did my first dive.
Relax, and have some fun
Safe diving
JohnF