It depends more on weight than on height. Your weight for the horse seems appropriate, but you are a very tall person. Being tall can have its advantages, because you can sit a buck better and have more use of your legs. But if you are looking to be competitive you need a proportional horse. It paints a prettier picture for showing. If you feel you have not only outgrown your horse physically, but skill-wise as well, then you probably should look for a horse above the 15.1hh range. If you are happy and your horse is comfortable, then enjoy your horse until you reach that point.