Pulling your horse's head down does not make him collected.
This is why the underside of his neck is more developed then the top (upside down neck). This is why his canter is strung out and the only time he has impulsion is when he's in the extended canter. Changing the position of the head and neck, either with your hands or with gadgets--which you also had on him--does not make a horse collected.
'But my trainer says'--Think for yourself. How does changing the angle of a horse's head and neck make him take more weight on his hind legs? Or lift his back? Or step further underneath himself? It doesn't.
This is why your horse is so unbalanced--he is too busy fighting your hands. He is not 'on the bit' or 'in an outline', he is fighting to poke his head up the entire time. Why?
Sit in your desk at home, and put your chin on your chest. Now imagine someone was forcing you to put it there (And that is what you are doing by holding the reins low and putting him into drawreins/sidereins/etc.). Now keep it there for five minutes, ten minutes, etc.
Don't you want to put your head up too? Doesn't it hurt?
Horses are the same way. When you force their muscles to hold a position that they are not in the right shape to hold, it hurts
. So not only are you making him tense and destroying any chance at collection, his neck and head are crying out for relief.
I have a horse like this myself (...it's always the chestnut ones that are the difficult ones!). I've done all the things you're doing now. All of his 'pretty trot' and 'beautiful canter'--if you change the way you are riding him, this horse will turn into something you can't even imagine. He has potential--but at the moment, you're going about the wrong ways to bring it out of him. You're stifling his ability rather then helping it form.
I've done it. It's hard to change. But if you can do it for your horse, you'll be amazed at what he can do--and how he'll take care of you. He dumped you over the one jump because he thought to himself, 'you know what, putting this much effort into this jump is just not worth it.' If you start riding him where you take into consideration that he needs to stretch, that you don't have to pull on his face, riding won't be a chore to him. He won't fight you or be 'stubborn' or 'stupid' or have 'bad days'. He'll think, 'you know what, it IS worth the effort because she asked!'
He has it in him. But do you?
The first thing you need to do is let go of his face. He needs to learn what soft hands are; he needs to learn what 'correct contact' is. Correct contact is when the horse WANTS to take the slack out of the reins. This is a video of my chestnut crazy horse, that I had been told by world trainers, multiple times, to take to the slaughter house because the only thing he was good for was running 90 miles an hour and bolting with riders: http://vimeo.com/4236051
(the embed did not work!)
You see how he wants to follow with his head as I put the reins forward? He likes contact--he is the one taking all of the slack out of the reins. He doesn't fight with me. You see how slow his legs move, but how we're still covering ground? This is what relaxation does to their legs, they push and they take weight but they don't rush. This was a horse that had a 'giraffe neck' that I've actually had a root canal done because he smacked me in the face with it whilst we were jumping--look what happened when I stopped pulling on the reins and demanding he carry an outline.
I don't ever pull on his head--because collection has nothing
to do with a bit of metal in their mouth. If I put something in your mouth and pull on it, does it make you do things with your legs (except kick me, hehe?)? Horses are NO different.
The first thing you need to do is work on long and low with your guy. He'll calm down, he'll learn how to accept the bit as long as your hands are soft, and he'll learn how to release his back and swing. I have a post in a blog written about long and low and how to do it and what it should look like, but I won't post it unless you would like to read. I've already written a lot and you might disregard it all, I have no idea.
You're horse has a lot of potential--do it for him.