WHY?!?! i did terrible, TERRIBLE! I am mad...help!?!?

I have next to no experience with horses. I've been on horseback once and was terrified, I get vertigo so it was awful. Anyway, you had a bad day and I know about those. Blu is not going to get shot. You're home now, do the cool-down routine with her, rubbing or whatever, talk to her and let her know you still love her. It's behind you now. Take a minute to settle your own nerves, a warm bath or a hot cup of tea, or both. Forget the idiot at the show. There are idiots all over and the best thing is to ignore them. Keep working with Blu, see if you can get her into show situations, not necessarily at the time of the show, but just the general atmosphere, so she can be accustomed to how it feels to be there. Visit shows you haven't entered, give her a feel for it. Then when she has to perform she will be more relaxed, and so will you. And you will show the idiot that you can do it, you and Blu, and he can just shoot at ants on the wall in his barn or something. Best of luck with your showing.