I'm not sure what you did wrong, if anything. This is another of those problems where a cut and dried answer won't cut it. If I was there I'd be able to actually see what is going on and who is doing what, but I'm not so about the best I can do is jabber on some and hopefully something may click.
As far as being at a loss as to what went wrong "today"...if it did happen on one particular day I'd tell you not to worry about until it happened again, because on any given day anyone can be beat and these things occur on occasion. However, the rest of your letter says this has been building for sometime.
I feel especially compelled to tell you up front, based on your relative lack of experience, to be careful with this horse. In my opinion and observation, for what it is worth, Appendix Quarter Horses, in an extremely high percentage of cases, require rather experienced riders.
Quarter horses are slow-twitch fibered horses. They are bred for long work days that require occasional bursts of speed (cows aren't really all that fast, you know). In other words, they are bred for economy of energy. TBs, for the most part are bred for flat out speed over greater distances. In other words they are bred for expending energy. When Quarter Horses are bred to TBs you are breeding a slow twitch horse to a high twitch horse and these two qualities often war with each other. You get a horse who can be calm and lackadaisical one moment, or rearing (sic on purpose) to go the next. Predictability and rationality is not the Appendix's strong suit.
Years ago, in my more traditional cowboy days I was talking with a Utah friend of mine, one of the most proficient traditional cowboy horse breakers I have ever met and as we discussed a particular technique he said, "unless, it's an Appendix Quarter Horse, they flop around on the ground like a fish. I have never had one that didn't fight it." He went on to tell me his wife was the emergency room director at the area hospital, "This is cowboy country, we get a lot of horse injuries, I tell my wife to keep track of the breed as much as she can. Most of them are Appendix bred."
I never said a word, his experiences with them paralleled mine. I thought I was the only one who felt that way about them.
For those of you who are reading this, no need to email and tell me that you have or know of an Appendix who *ISN'T* that way because I have heard it all before.
Now then, does that mean the situation is hopeless? No, not at all. Just that you'll have to be more careful and diligent. Always wear a helmet and other safety wear.
First of all, you have to get the horse's respect. I'd suggest reading Lightning Is An Unlikely Cause Of Loading Problem because in reality you have the same problem - namely establishing a connection and maintaining and moving the control of the horse. When you have read it, get a copy of the bonder and read it over and over until you understand it and can do it in your sleep, then put Paco through it and see what difference it makes.
Once the respect issue is dealt with and he still acts up, the next course of action is to look for physical causes. He may very well have some physical things going on, but from the things you report, my gut feeling is disrespect and the insecurity of being connected is the biggest part of the problem.
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