First, congratulations on taking on a mature, unstated thoroughbred stallion and getting to where you are now. Your issues when you work with more pace (in the canter) and when you add other challenges, such as jumps and poles, are most likely due to your horse’s lack of sufficient balance. He spent six years finding his own balance without a rider and it isn’t surprising that he has trouble now when he attempts new things. The good news is that his maturity permits you to continue on with his training according to how quickly he picks things up. A younger horse has a much shorter attention span and will tire, both mentally and physically, much quicker.
I suggest going back to the German Training Scale in your work with your horse. The Scale ranks the following, in order of priority: Rhythm, Relaxation, Contact/Connection, Impulsion/Forwardness, Straightness, and Collection. The goal is to use the Scale both on a moment-to-moment basis in your riding, as well as the basis for your overall game plan for producing an educated horse. As I’m sure you’ve learned along the way, bringing a horse along is never a smooth and consistent venture; you’ll have periods of progress along with some plateaus or even going backward a bit. You use the Training Scale as a reminder that, as you ask for the next step, you assure that you don’t lose any of the earlier steps along the way.
Some horses seem born with a natural balance at the canter or gallop, but for every one that it comes easy to, there are lots more that will take considerable time until they are capable of carrying the both of you in a comfortable, balanced, adjustable canter. Managing it with jumps or even poles in the way makes it even harder. So many training issues turn into various forms of objectionable behavior when a rider tries to force a horse to do something he simply doesn’t have the capability of doing yet.
I’d suggest for now you go back to simpler work where you can firmly establish the first three levels of the Training Scale at the walk and then the trot. Limit your work at the canter to doing medium sized circles at whatever pace that enables your horse to establish a rhythm and learn to relax. Limit the use of your reins and even your leg to only what you need to maintain the canter and stay on a circle. Meanwhile, you will work on getting a good forward rhythmical walk and letting your horse learn that light contact with his mouth and accepting your lower leg softly on his sides is a good thing and doesn’t cause him to lose either the rhythm or relaxation. Add in some poles on the ground and then create the same attitude in the trot. Only when he is consistent—and you have the tools to re-establish rhythm, relaxation, and non-demanding contact (in that order) every time you lose them—should you begin to teach him that application of leg aid means to go forward each and every time you apply it. Only when he thoroughly understands what you are asking, and your undemanding circles at the canter have lead to rhythm and relaxation, should you begin to introduce contact and leg aids at that gait. Remember, when you attempt the next step on the ladder, be sure you don’t lose the earlier attitude. Only when your horse is comfortable with the first four steps can you even think about making your horse straight and adjustable with your aids.
Looking for collection—the ‘sitting’ you are referring to—is far in the future for any green horse, with the exception of brief moments such as the take-off at small jumps from the trot and the first step or two in a walk to canter transition when he is doing them correctly.
Even though your horse is still working on his canter, it doesn’t mean you can’t include some ground poles, small jumps from he trot, and trot-in gymnastics in your rides. They will help him in his balance. Keep your aids very simple; just enough leading rein to be sure your horse learns to go to the center of any obstacle. You want him to approach them with a relaxed attitude and return to the slow relaxed and rhythmical gait that you started on before you finish every exercise. Any more ‘help’ from you isn’t necessary and won’t give him the opportunity to learn how to deal with obstacles in a calm and focused manner. When you are not distracting him with pulling or kicking in front of the obstacle, he will be able to learn from the mistakes he makes.
Keep your work quiet and fun for your horse by avoiding too much repetition. Limit asking for the same thing over and over again. Two or three times is enough, then change it up or go on to something different and perhaps come back to the first exercise later in your ride. Riders thrive on repeating things over and over again—horses tend to give up trying when their attempts are only met with yet another ‘ask.’