Go back to basics! Get someone to lunge you. Let the horse go slow trot and let the reins down and get hold of your own hips. Make sure you're looking up, head straight, make sure you're sat on your seatbones and use your core to absorb the horses movement, just go with it! Do it as often as you can and this should help.
There is a lot of exercises you can do in walk to open your hips and make yourself supple.
When you're on board, if you bring your knees right up so that they are touching, as high as you can, then open your knees outwards and push your leg back down, it really opens up your hip. Do it a few times and you'll feel the difference!
Read more at http://www.horseandhound.co.uk/forums/showthread.php?126101-Exercises-for-riders-to-open-the-hips&s=0b2bad4bf9724fffcbff6388f694c704#xzT3JeShlz5V4JyZ.99
Do lots of sitting trot without stirrups and learn how to absorb the bounce and move your hips with the movement of your horse. And tons of practice without stirrups will help! Fully utilize no stirrup november, and even do no stirrups when it’s not november. I try to do no stirrups every time I