Can anyone help me with my canter?

I've been for an hour a week for about 4 months now. My problem is the canter, I have the correct leg aids and get horses into the canter, but I'm so inconsistent - a stubborn horse I managed to get into canter fine, but couldn't keep her over poles and then a horse that was ridiculously forward would go into trot perfectly but seemed to be ignoring my canter aids. I also have trouble staying in my stirrups and so find it hard to put my outside leg back and kick with my inside at the same time.. any tips? Because Its really starting to frustrate me.