Tricky horse on ground but amazing when riding her?

I go to the stables every saturday fortnight and there is a mare there called Polka. She is a black new forest pony. She is my favourite there, really responsive when i ride her, great personality and a big softy most of the time haha :P Theres just 2 things i need help with. 1. She loves to trot, but i need to get better at my canter, its getting better to get into canter, but most of the time i wobble about, and loose controll Only slightly though and then she will go back to trot when i dont want her to. The second thing is, when i am leading her from the ground, she is so grumpy and wont move. I always need someone to get her going by tapping her bum, and as much as her pull her and talk to her she wont budge. So i need help with those 2 things. I need some answers soon cos saturday im going riding and at the stables all day.