As a rider of 35 years or more during which time I have ridden a 100 or more horses - who knows how many - including riding in several different countries
I believe that a serious fall, especially one invoking shock and maybe concussion, can bring about what I call "Post Traumatic Fall Disorder".
We do not ride with our conscious brains, we learn to ride by rote - constant repetition. The instant responses we need to keep up on a horse's back are taken in by our sub conscious brain. If a horse shies, then we have to have responded to that shy before in some cases our conscious brain has recognised that a shy has taken place. When driving a car, if another car comes up from the side, then we have braked or turned (or both) before we have visually recognised the situation.
The problem is that our sub conscious brain has reacted to a threat to the body's well being. It is well possible in regular horse riding that similar threats have been faced before but since nothing too serious has happened previously then the brain has shrugged it off. But once serious physical damage has been done - eg if the spine has been bruised or the head banged - then the sub conscious starts to think for itself. It sends out little signals and says - "Oi take more care".
The problem with horse riding is that 'tension' is an enemy. If the rider grips or goes rigid then the body's ability to react in a split second is negated.
The body can't absorb the stresses.
There is a second problem too, the horse can sense the tension in the rider.
Then the horse starts to ask itself why the rider is tense "WHat's going on?" asks the horse - "should I be worried too".
The psychologist might say that the human brain is as good at remembering as the horse is - and the human brain remembers other fears, unrelated to horse riding, which may also be worrying the individual.
How does one cope? Well first one accepts that there is a problem.
Then one learns as much as possible about relaxation techniques.
Then one goes back to the training arena to learn to sit and relax on a horse - one will need professional help for this.
Then one asks oneself if this horse riding is what one really wants to do.
Then one allows 'time' to do its healing magic.