A matter of trust

So here I am in mid life, starting over with a new horse and learning a new riding discipline. I have ridden for over 30 years in all sorts of situations but find myself feeling like a beginner again. I purchased Harley just a year ago as a green 4 year old. He was well started in reining but lacked rate and patience in his manuevers. My backround was in halter horses and trail riding for the most part so what an adventure I was in for. I have been riding him steady for the most part and working with a trainer for lessons once or twice a month. We have made some great progress and I feel like he is the horse for me. I have shown him at ranch horse shows this past summer and we continue to improve but I still lack the ability to get him relaxed. Herein is the problem I think, I simply do not trust him and he doesn't trust me. It is nothing I can put my finger on per say, just a lack of trust. He has never offered to run off or buck uncontrollably, he just has such a big motor and is so lightning quick in his maneuvers. My ultimate goal for us is to compete in working cow horse with no particular time frame in mind.
I keep thinking the more I ride him the better it will get, but so far I am not making a whole lot of progress. I am open to suggestions for anyone who may have encountered this or can think of things to better our relationship together. I suppose it is just like with people, a trust must be built upon and takes a while to establish.