You need some help here. How anout the BO. Not to train, to teach YOU. Maybe your vet or farrier knows somebody who would be willing to show you what and how.
One bright spot here, it doesn’t sound like he spun and double barreled you, Sounds like you were just “ collateral damage” from his disobedient spook. If he really meant to go after you, you’d have been in the ER with broken ribs and possibly internal injuries.
Legend just helps the joints lubricate themselves. Sometimes we learn they were only quiet and well behaved because they hurt too much to get silly. Thafs why it’s so important to teach and enforce good manners and practice proper handling. You have some great suggestions upthread but you do need some hands on training with a real person to really teach you safe ground handling
All ten commandments of leading a horse are you MUST stay at their shoulder. They MUST also keep you at their shoulder, no speed up, no slow down, no turn away. That is what you are having trouble with because you have no way to correct him and to add more pressure if his mind wanders. Just as important is promptly releasing that pressure, he keeps miving away- you keep pulling- he keeps pulling. He’s bigger, he’s going to win.
Get somebody to spend 15 minutes with you a couple of times a week. Don’t use a chain until you know how to rig and use it, right now, it’s unintentional sloppy handling and loss of focus. Mishandle a chain on him and he will get defensive and scared. Make things worse.
This is fixable but please get somebody to help you.