Babies startle at loud sounds; who knows if your parents had a car that backfired a lot and woke you, etc. LOL Maybe you were along with others firing guns or hunting, etc. and didn't like it at first but were too young to really remember the details.

I had a good friend who grew up in a rough neighborhood and she and I were travelling through a wealthy neighborhood on our way to a specific store and we were stopped at a light with a country club on the corner and a gun went off. She hit the floor of my car but I was merely puzzled because there was no way a gun was going to be fired in the middle of the day at that location to my way of thinking. I worked at figuring it out and finally realized it had been the country clubs swimming pool starter's gun for swim races!

Did you see the movie Sea Biscuit? They had to work to get him use to the bell ringing. Your instructor had to work with you at first to get you use to guns firing and here one fired when you weren't expecting it after all that time. I don't think we get "over" what scares us, we just get so we can function better in spite of the fear but in a sudden incident like you just had, that's like an emotional trigger is and you had no time to "prepare" yourself.