My AQHA mare
I have a horse that is very much like the horse you describe. Let me explain...
We moved into the country to raise our kids away from the perils of the city. Nice dream, but it didn't work out the way we planned. THAT is a long story I won't bore you with, but we acquired horses as part of the plan. At one time we had seven horses. Like everything else, time changes things and we are down to two now. During our adventure we enjoyed family trail rides and field picnics. We also hosted a horse focused 4H group for a year or two.
Ten years ago I bought a motorcycle after not having ridden for more than 20 years. I had stopped riding motorcycles when my family began to grow. Now that I am an "empty-nester," I felt it was ok to ride again. However, that meant that the horses we had acquired became lawn ornaments. We feed every day and lay hands on all of our animals, just being responsible, but we just don't take the time to tack up when we can simply turn a key and ride.
Fast forward to this week. After three different motorcycles (I still have my dream bike in the garage and rode to work this week) I found my desire to ride my horse again. More importantly, my wife announced that she wanted to start riding again. That's why I'm here...because I become obsessed with what ever I get involved in and this forum has become a tool that I use to educate myself about trends and tools.
I rode my horse for the first time, this year, last weekend. I hope to ride again tomorrow, weather permitting.
When we led the 4H group, we taught that the most important point that every young person learn: NEVER take anything about your horse for granted. That's when you will get hurt. NEVER let your guard down. I carry that lesson in my head, constantly, when I am working around our horses, and riding my mare. I love her, but she's hot and strong willed. After not having been ridden for almost 12 months, she picked up on every single cue I provided. She is absolutely wonderful, but she still tests me every time, just to be sure I've brought my A game. She wants to be sure I'm paying attention. At 55 years old, I am also very very careful about placing myself in a position of risk. I always remember that I don't bounce as well as I did when I was much younger. Therefore, I must always be as attuned to the circumstances at hand. I have come off my mare in the past, and I've been lucky to have not been hurt. But riding horses and being around horses include an amount of risk. I am very aware of that and I hold that first lesson very high in my mind when ever I am in the field, riding or feeding.
Addressing your incident: I don't believe that you did anything wrong or that you will always be able to know why this or any horse does what ever they do. The best you can do is to be on guard, ride with your best technique, and always be aware. Even then, you can be surprised. That's the nature of our sport.
I am sorry you are hurt. I have cracked a rib and I would agree with your assessment: ribs are one of the most painful things I have EVER experienced. Of course, being a guy, I haven't given birth, but I have had various surgeries and none of those hurt as much as a cracked rib. EVERY move, every breath, every moment is pain, for a while. But it will get better.
I would not give up on this horse. Based on the information that you provide, this horse is a good match. The positive things that you can list: you have some experience with him. Not much, but some. Any other horse will be like starting over and you already have the benefit of the limited amount of experience you have already acquired. Plus, you have the benefit of a trainer that has ridden this horse and agreed with the quality and basic mind set of the horse. Maybe an easier way to state my opinion is to quote an old saying: "A bird in the hand..." You have the benefit of a "known quantity." I will caution you, though, that I am bias because I love quarter horses. The can do attitude combined with the stocky, chunky, muscley build packaged in an athletic body. I just love 'em.
So, heal up, feel the warm sunshine on your face, enjoy your new horse, and ride with awareness. Take nothing for granted, wear your helmet and enjoy. I believe that you have found a diamond in the rough. A little rough that can be worked out. He hasn't been ridden for a while so he's not used to listening to someone else's will or instruction. That can be changed and you can develop into a wonderful team; horse and rider, friends. It's time to cowboy up. Let's ride!
"The road goes ever on and on, down from the door where it began. Now far ahead the road has gone, and I must follow, if I can." J.R.R. Tolkien