Well I was going to say try no stirrups..maybe try it on a more experienced horse? Or try and get a lunge lesson, it really makes a difference! Maybe you could try just walking and trotting without stirrups for now? It would really help with your balance.
Erm, I guess just try and relax and don't brace yourself against the movement, put a hand on the pommel if you feel unbalanced. Lunge lessons are great 'coz you can just think about yourself not the horse.
Good luck.. it will come in time and with practice. [: