If you have a horse that is suddenly refusing to let you ride him, he won't walk on a lead and suddenly decides to go into a full trot when you want him to stop, you don't have a stubborn horse or a horse that has forgotten his training, you have a very clever horse that is testing your dominance.

First of all rule out anything physical that is causing your horse discomfort or pain. Many horses will act out in an effort to alleviate pain and also call your attention to it.

Every horse is an individual and they all have different levels of aggression. Some horses will never challenge your role as "top horse" and others will periodically throw a proverbial horseshoe in the works just to see how you react and if you are still in charge. Also, firmly but gently remain in control. Never yell or strike your horse, he will just decide you want to cause him harm and all the training you have done will be immediately undone and your horse may or may not trust you ever again.

A good way to remind your horse who is in charge, is to revisit early training such as ground manners and leading. When he listens and responds, praise him and give him a treat if he does exceedingly well. Horses love both and they are effective teaching methods. If he disobeys, gently repeat the command and continue to do so unless you lose his interest or he becomes agitated. If you feel like you're losing your patience, end the lesson. A horse will pick up very quickly if you're feeling stressed and many horses will mirror your emotions.

Your horse will need frequent training during the process of reasserting your dominance,don't waiver or you'll teach him that he can be in charge "sometimes". Stick to praise and rewards and you will have your horse retrained in no time.