The way you feel is perfectly understandable. I think you are being reasonable and trying to do the right thing. I think you are doing it better than a lot of 20 year-olds. Despite being embarrassed, you didn't get mad and do something stupid.
It's a little bit tough to tell if you really did anything stupid. Were the rules clearly posted? Did they have signs posted that said something to the effect of, there are cease fires, do not go downrange until all weapons are clear and the all-clear is given? If there WERE, then you do need to pay more attention. If there WEREN'T, than it is the responsibility of those who run the range, be it this OD cop or whomever, to take reasonable measures to make sure that everyone who enters the range knows the rules. Stop everyone at the gate. Tell them, "We have a cease-fire every 15 minutes. The next one is in 8." Even something like that. I THINK, that if you are going to expect people to know when to know when the range is supposed to be clear, then you must have safeties and clear commands. If you don't, you are going to have to leave people to their common sense.
I think over time you will get over the cop's dressing-down. He wasn't a total jerk to you, and he cares enough to stop people who are doing something unsafe. He could have been a lot worse.