Hey so I hope you have it down by now, but if not I could help you a bit. I just went through with a trainer on the same issue with my horse that had never been ridden. He tried to toss me numerous times and succeeded once- I ended up being stepped on that time. The problem might be that your horse just doesnt know what you want it to do. That was Triggers problem. After a few failed attempts, we put him on a line and attempted to get him to lope, he flipped out because he had no clue what we were asking. It took about four weeks working him on the line until he was calmly picking up the lope- without me on him. Now he is in a snaffle bit, split reins, and I wear spurs- just as an extra cue if leg pressure isn't enough on that day(note: only use spurs if you have been trained on their use). He now lopes for me on command, but it took me almost three months to reach this point. As for seat and all- sit your butt as far in the seat as possible, lean back, but not so much its unnatural, heels down, and rock your hips with the movement of the horse, it helps them when you move along with them. I hope that helps!