Hi so sorry to hear about your frustration. All of the things you have mentioned point to lymes. Rushing at the canter, not wanting to go forward, bute and prevecox having minimal effect. Lymes can lie dormant while the horse is not in work or in little work and the present as soon as you up the training load, travel load or show load as these are all stressful. I would have him tested to rule this out. I would also recommend the test that tells you what his lymes titer actually is. It is more expensive but worth it. Other things you might want to consider are epm and kissing spine. If the test is positive you may want to consider the Smartpak Protect Ultra to help boost his immune system and minimize internal stress. Good luck with your horse.