controlling trot after cantering

I'm new here and I ride dressage on a 10 yo tb gelding in canton, ga. I started riding as an adult and bought my horse 2 years later and its been 2 years since then. He is a well trained but sensitive horse, easily feeds off of my nerves. anyhow, for a while I was doing too little cantering. He had some pain issues and every canter degeneraged into a buck or a bolt. The pain is fixed and we are both a little anxious when it comes to the canter. for the last few days I have been pushing my self to canter more and more, and this has been good for both of us.

here is the problem. I have trouble controlling the pace of his trot after we canter. he will race at the trot and try to break into canter - and of course this makes me nervous and that makes him nervous. I will then do trot walk trot transitions, and that brings him back somewhat. I also figured out that if I squeeze both thighs, he slows quite a bit. Half halts at this point seem to piss him off - I may lack coordination at this point because of his racing. How do all of you controll speed within a gait? any advice how to improve this?

Thanks, Harriet