I feel your pain, hon. Rebel's trot is the devil and nearly impossible for me to sit without dramatically leaning back and gripping the cantle, and god forbid I get into his lope.
First off, you really shouldn't ahve your stirrups as high as they'll go. You aren't a jockey. You should be able to stand up in the stirrups and leave about four inches of room between you and the seat of the saddle. Keep the ball of your foot placed on the stirrup with your heels down. And when he picks up the trot just lean backwards in an overly dramatic way. When I was learning to ride as a little kid I had a big arab mare named Angel that had a terrible trot, and I used to hold onto the cantle until I got the hang of it.
And hell, maybe you need to post. Some trots are just meant to be posted to. Just get into a rythym and move up and down with his stride. It makes the trot a lot more comfortable.