I agree with loosie
in that this may have become a learned behavior (ruling out physical problems first such as teeth or back problems or tack fit). Does he do this at all gaits or just at the trot or lope?
I would start by trying to isolate the problem by lunging him for a bit then one by one introduce his tack. Start with just the bridle then the saddle and lastly someone in the saddle (but your horse being worked by the person lunging not the rider).
I don't know what bit you are using but would start with a snaffle in any case. Unless you are doing advanced work or showing with your horse, a snaffle should be all he needs for general riding. Too often we use stronger bits and other artificial aids in the place of good training and that leads to behavioral problems.
In addition, and after all the physical and tack problems have been eliminated, I would only work your horse at a walk. Let him become comfortable and confident in you. Be light with your hands. At first let him wonder around the arena at his own pace and direction - just be the passenger. Little by little (and this does not have to be the same day) give him some direction. Let him build confidence. As you move to a different gait don't be surprised that he reverts. Just let him get comfortable and confident. He'll get there for you.