Perception - to hear the unspoken

Easter holiday and wonderful northern spring means that the blog has had to wait until sunset. The challenge is to capture the good days as a buffer for days of another kind.
In August 2010, I was out riding my anglo-arabian mare. We were out in the woods and rode on nice, gallop friendly sandy pine moore paths and I was so happy. She was alert, responsive and felt wonderful. One day I climbed her up a slight slope, four steps, we took the little slope twice - eight steps all in all.
We walked home and she went from calm and at ease to toss her head and stepping, she became generally annoying. I halted and thought that she would come to her senses, but she began to back up, and in the middle of a forced step back I felt how she lost her left hind leg. I got off her back and started walking home. Confused.
A few years later an acquaintance told me that she, after having taken a course on horses and bits, realized that the bit she liked to use when riding caused the most pain in the horse's mouth. It was incomprehensible to her, she felt that the horse was so soft and smooth in riding.
Another told me that they had discovered that the horse had lesions in the mouth but in riding the horse had felt soft and flexible. Another told me that the horse worked like a dream just to show lameness the day after.
Once is once but twice, three times, four times ... I know that this is not a statistically significant number, but never the less it raises a question in me. How well can we read horse?
Horses have utilized silence as their survival strategy, they are experts at hiding pain and the effects pain have on their bodies. Could it be that all the horses in these examples had showed small signals that something was wrong and since we as riders did not notice them, they moved within their frame of pain, and we considered them pliable and soft?
In my own case, I had taken her outside her framework under and she became very vocal in her behavior.
Had it been possible to discover pain reactions of the horses in the examples above by using a heart rate monitor? Were there some small indications from the horses' side that we missed, were there some big?
It is said that before you judge a horse to be disobediet, you should ask yourself if it understands what you ask for, if it has been trained to do what you ask for, or if it is physically fit to do what you ask for. You can tell by my wording that the way I reasoned in 2010 was disobedience - she was annoying and needed to come to her senses. The fact that it was the pain became apparent to me later, but at the moment disobedience was a closer option.
I have learned a lesson from the incident with my mare. Now I strive to ask me if it can be any of the other reasons that prevent the horse to do what I want instead of disobedience. I'm trying to train my perception, my ability to read horses better.
My new mare, whose feet carried me to AEP, enjoys the benefit of my homework and is also a good teacher. She signals in an ascending scale if I get too close to the edge of her frame of pain. In our training I don't not ask for any gait other than those she chooses to take in the pasture (the scare canter is not included). She has just recently begun to take a trot based on self carriage rather than a trot she jumped into. So we have now added short trot repetitons in our walks. We can take walks up to 1.5 hours, when we started she wanted to turn around after 10 minutes which added up to 20 minutes walk.
When we worked from the ground in a shoulder-in like move one step with left hind was enough in the beginning, one more and she gently nibbled my hand. Now we can take a series of steps in the corners. I propose a move and she adopts or rejects. It is not a question of obedience or disobedience to me, I trust that she knows what she is capable of doing and if I ask for more than she can manage so she shows me.
I see that she trained when she moves in the pasture. She's doing nice rollbacks on the narrow path where the gelding, who can not, must detour into the deep snow. The trot she offers now I long to ride. I really long to ride her, but I'm willing to give her the time she needs to heal. We do this together, she and I, we are a team.