I suggest the same, leave the stirrup, only sit forward, let your legs fall downward. They should be naturally in the same position that with long stirrup, just slightly flexed at the knee. Let your shoulders bach and relax. Don't hang on the bridle, if you feel unsure grab a little mane. Try to go with the horse movement. And most of all, try to worry as little as possible and keep your neck straight looking forward, do not crouch and look at the horse head.this is what I try to do. Now, I have a friesian and he is very high trotting, so it is a bit harder.