I did not watch your video, I have dial up.
In any training, you should know what you want instead of what they are currently doing.
So don't just say "no", give him something else to do in it's place.
In this case "back up".
Not only do you get him to get off your heels, but he learns the word "back"
Just keep asking him to back off.
I have never had one that didn't listen after the first day.
Consistency is always the key.
If your standing grab the lead and ask him back.(short quick tug on lead) If he follows you ask him back again.
Doesn't take very long for them to figure it out.
If your walking away and he is right on your heels stop immediately and ask him back.
Then proceed, if he does it again, do the same thing.
He should follow you at a safe distance weather you walk backward or forward.
Longing, To keep him out in the circle, throw your slack at his side. If I am not using a line I'll use the whip and hold it straight out between me and the horse, so he can't come in. Then if you ask for more speed, he will have to move out. Then slow him down on the rail by slow body movement or calm voice.
Watched 1 min.
First of all, when you are walking, he is leading the way. No no no, get him off you. If something were to spook him on the right , he'd be on top of you. Same thing in the middle of the arena. You look as if you are in front of him.
Lunging is a driving move, you should be towards his hind end. If you want to stop or slow him, then step toward his front end. He is learning from you body language. Also, you dropped the slack. Hold the slack in your other hand , you should not be stepping over it while working him. If he were to turn, his leg is going to be caught in it.