Feeling frustrated with my canter seat?

I have been riding for a long time, and most of the horses i have ridden were on the lazy side, as in didnt want to keep cantering or didnt want to get into it. Last summer I had a BAD instructor put me on a horse that would gallop around the ring and she didnt care how to slow her down , or how i looked, just that she got that 40 dollars for the "lesson" so i learned to pinch hard with my knees get all over the place and not slow down the horse because that is what she told me to do. I have owned my own horse for many months now and I have slowly been fixing my bad habits like pinching with my knees and keeping my heels down getting my leg to stop swinging, and my horse is a challenge. Hes sort of green and lately it seems when i canter he slowly starts going faster and faster, until we are galloping around the ring, BUT if i ride him in a 2 point at the canter he goes perfect and slow all the way around the ring, and trust me his saddle fits great so its not that. I asked a trainer at my barn what i was doing wrong she said when i was sitting my seat was asking him for more canter when i should be lighter in my seat. when i get lighter in my seat my leg starts swinging and its like i cant make everything stay in one place, and i used to look PERFECT when cantering and now i look all over the place and my friends have noticed to. The trainer at that barn doesnt have a lot of time because she works until 6 everyday, and she has her own horses. I know i can do it but how do i get all the peices together? somehow i used to do it effertlessly but now i am starting to make my horse gallop without even realizing it and im getting all over the place. And my barn is wicked busy so its hard to get a trainers free moment.