Make sure you're sitting on your bottom and not you fork. A good tip which my OH makes me do a lot:
Feet out of stirrups, bring your knees up to the pommel so they touch each other, bring them right up. You should feel your bottom coming underneath you. Then slowly put each leg back down and you should find you keep your leg position a bit better. Takes a bit of practise and does hurt!
if u cant keep ur heels down... do u want 2 take myn off me, my trainer keep tellin me heels up.... i guess she teachin me dressage and i bin a sjr and hunter 4 2 long lol
I'm with the no stirrups school of thought, including rising without stirrups. Another good one for balance is standing in your stirrups for a circuit of the school in walk and then in trot. (hard work but very good)
Obviously it will all take time and work etc etc but my legs go back when jumping and my boy hates it. He likes them to be kept by the girth in that lovely position all the eventers seem to keep which I cant! I now jump in those velcro stirrup strap things (cant remember the exact name now) but they are brilliant for me and might work as a quick fix for you if you need them and they are only about £8.