where abouts in aus are you?
unfortunately, a lot of pony club people can be like that. i find a lot of them very snotty lol (and im moving to a pony club in a couple of weeks

) anyhoo, try and remember you arent there to please them rather you are there for you and your horse. i know its hard but you really need to ignore what these girls are saying. maybe your parents could get you your own trainer if the PC instructor wont help you or, maybe they can have a quiet word with her and let her know that you are unconfident and that you need some help.
if you are feeling unconfident the only way you can get past that is by practice, practice, practice. maybe you can try riding when the other girls arent there and slowly build yourself up and next time they see you ride they will be all surprised and eating their own words :)
keep your chin up and please dont let them get to you. if these girls are so mean as to say such awful things then they dont deserve your time or consideration. maybe you could try posting some questions about your exact problems and people here could help you work through them. you have mentioned a little about your issues but if we could hear each problem in detail im sure between all of us we can get you going nicely and hopefully better than the little pony club snots :)