Such a sad loss of a young life :( Unfortanetly as everybody said, we have to try and live life without fear, with the freedom to enjoy life for what it is. I know it won't make you feel any better at the moment, but remember that you have as much a chance of getting into a car accident as you do falling off your horse. That horse would not have wanted to hurt her, nobody will ever understand why this happened but she is somewhere else in a comfortable place and not in any pain.
My thoughts are with you and her family.
Stop for a minute, open your mind, learn. You may not agree with what I say, I may not agree with what you say but we will both learn something new.