flying advice needed

Hi, The last time I flew on a plane it was a rough landing and my back was thrown forward and my head came down very close to my knees which resulted in a whiplash of the lower spine. I am surprised that airline seats do not have shoulder belts, only lapbelts. I would like to fly again, and was wondering if you know of a portable shoulder seat belt that I could buy and take on the plane with me. Or if you have experience with any airlines that offer shoulder belts to people with back problems. Thanks!