I hope that you will read my post. I was disappointed at some of the responses you received and will admit that I didn't read all of them before replying.
First, you have a very serious issue here - this horse has hurt you severely 3 times and so we have to take a look at the entire situation that could have caused his reactions and then what options you have.
His head tossing, then his violently throwing you is an indication of pain. Albeit there are going to be some training issues here first, any respectable trainer is going to ask you the same questions that I am about to because no amount of training is going to make him succumb to the pain he is feeling. So I would first advise you to have a vet over and have him check especially his mouth.
It may be that he needs to have his teeth floated, or perhaps he has a chronic abscess that his bit is irritating. He may have wolf teeth that are creating so much pain that he has to get you off of him.
So why would he do this at the canter and not the trot or walk? well, your riding skills could be coming into play here and aggravating an issue that he could tolerate at the trot or walk. Without realizing it, you may be putting undue pressure on the reins at the canter. You are probably scared to canter him and with just cause. So your fear, whether or not you actually realize it, can be making you grip the reins, tense up and cause pressure on his mouth. If he has a painful mouth this pressure is going to exasperate the pain. It would be interesting to see if he could be canted on a bitless bridle, hackamore, or just a halter with reins attached.
If it is not his mouth, then it could be his back. Again, the head tossing is an indication of pain and while ill fitting tack could be a culprit, it would not manifest itself in such a violent way unless there was an underlying problem already. And if he canters in the field fine, there's another indication that it has something to do with the connection between his tack, you, and his body.
Another pain aspect is his legs, and at 10 years old he could be showing signs of arthritis or joint disease. He could have suspensory injuries that you can't see. His willingness to trot and walk but not canter could indicate a strain on the suspensory ligament which would require surgery,about a year layup, and then reconditioning. Most suspensory injuries occur in the hind legs and become present when a horse refuses to do something he once did well. A violent reaction like this could be an indicator to a vet to check out the suspensory.
If it is his back or another joint in his legs that is affecting him, your vet may want to start joint injections, accupuncture, or treating him other ways such as Shock Wave Therapy, or a combo of treatments.
You need to get him vetted right away and have a competent rider be available for when the vet comes out. I do not suggest that you ride him for the vet because it is hard for him to explain to you what he sees without showing you it as it's happening.
Now, let's talk about you...
While I understand that you adore this horse, once you have him vetted you are going to need to ask yourself some hard questions. This horse sounds like he has done some serious damage to you physically and mentally. Time and time again I have seen people purchase horses far beyond their riding skills and have seen them get hurt, seriously hurt, and still keep believing that "he didn't mean to do it." Yes, they can be sweet as pie on the ground but that doesn't mean they are going to love you on their backs.
As riders we have to evaluate our own skills and be realistic about them. While you may be a very good rider, you may not be a very good rider for this horse. Should the vet give him a clean bill of health (if he does, get a second opinion fast) then you need to re-evaluate your position. A good trainer will correct his issues but that doesn't mean he is going to correct his issues with you.
I once worked with an abused gelding that no one could ride. He would rear at the mounting block, roll with you on his back if you did get mounted, and run with you on the trails. He would never soften at the poll and relax his back BUT he was an absolute doll in the field. So I was asked one day if I wanted to buy him but I didn't want to without riding him first.
The owners took him out, tacked him up and all the time were screaming at him, beating him and kicking him every time he moved. I didn't say anything, I just watched. I saw him panic at the mounting block which got him a swift kick in the belly at the girth. 45 minutes later the husband owner was on him, with his elbows in his ribs and the line between this poor horse's mouth and his hands was so tense I wanted to cry. About 5 minutes into the owner riding him with mixed signals (kicking him to go forward but with his hands so tight on the reins that he was pulling back on his mouth) the horse reared and spun. The owner stayed on long enough to crack him on his poll between his ears with the crop he also had.
I didn't ride the horse that day but started working with him every time I came to the barn. I would groom him, get him to relax, and walk him to the mounting block. There we would just stand and talk, while sharing a bunch of carrots. In a few days I got on him, bareback in a halter and lead and we walked comfortably around the ring and dismounted. In a few weeks we were trotting nicely, calmly and relaxed under tack. His teeth needed to be done badly and having the bit in his mouth was creating a lot of pain.
One day the owners had come out right when we got to the mounting block and saw me slide onto him easy, then watched us walk trot and canter in the ring and they were amazed. The next day I came out, heard the screaming and yelling from the ring and saw that the owners were trying to get on him. I couldn't believe that all my work could be undone in 10 minutes like that. When I went out there I told them I would buy him just to save him from them, but they upped the price on me from $500 to $3000 because they were so impressed by what he could do. As much as I regret it, I couldn't afford that amount of money and so I never rode him again. I wouldn't confuse him like that and be part of a mental cycle.
I'm telling you this story because I want you to understand that his issues are most likely medical and need to be addressed, but that they could also be about you too. After you vet him, find a trainer who will work with you and him and if you still can't get it right, then rehome him. But whatever you do, do not donate him to a therapy program. The last thing that you would want is a liability lawsuit because you knowing gave him away with violent issues.
But I can almost guarantee you that once you get him properly vetted, something will show up that is causing this, and after a bit of retraining he should be okay. It doesn't sound like he is viciously trying to hurt you. he's trying to tell you he's in pain and he needs you to help him by not asking him for too much.
Good luck. Feel free to Pm me if you have more questions.