Hindsight is 20/20

I had a little run in at work today with a drunk girl. Around my age and sporting a string bikini... and nothing else. The story would be better if I could say she shouldn't have been wearing this bikini but the thing looked like it was made for her 110lb tan and super fit body. She was a total knockout and maybe that made her more intimidating? I dunno, I guess I'm just justifying.
She threw money at my coworker and pointed to the gas pump and never took her eyes off her blackberry while her fingers were texting a mile a minute. He had to watch her walk out to her car to see what pump she was on. 5 minutes later she comes back in and exclaims that he stopped the pump at $8.00. He calmly tells her that that's all she gave him. She explodes and insists she gave him a twenty. Blah blah blah, I made him count down his cash register and I pulled up her picture on the security camera and showed her a clip of her obviously handing him more than one bill. She then changes her story to "I gave him a twenty and some ones.." Long story short- I left ten minutes after she did (clutching a piece of paper with my name and boss's phone number to complain to.) and I've been looking back on the situation all night wishing I had handled it differently. I should have called the cops and had her spoiled butt arrested for drunk driving. I should have stood up and talked to her with more authority. I should have taken a deep breath and made my hands stop shaking the whole time I was trying to resolve the situation.
Last week I had to confront 3 teenage boys for stuffing their pockets with candy. I was the only one in the store and they were totally oblivious to me watching the security cameras the whole time. They came to my register and I had to tell them to empty their pockets. I had 'em. Red handed. They were young. Innocent looking and timid. Why was my heart racing and voice shaking??
I daydream about standing up to rude customers or thieving kids. In the "dreams" my voice is clear. My hands don't shake. My cheeks don't burn bright red. I convince myself that the next time will be no big deal. That I will be calm and stern and full of authority. WHY doesn't it ever happen that way? I know exactly what I want and yet.. I can't seem to obtain it. I'm left with this hindsight and regret and daydreams of what could have been.