I also recommend teaching a one rein stop. Even for a reliable, seasoned schoolmaster this can be a lifesaving skill for both of you.
Firstly, though, I think much of your problem is a lack of exercise, "hot" feed, or a combination of the two. Daily turnout. Period. Preferably daily turnout and a workout, either on the lunge or under saddle. What does he eat? Unless he's emaciated, cut it way back, or strategically wean him off of it altogether. Horses can survive easily on free choice hay alone, and your guy sounds like the last thing he needs is the energy that a concetrate or grain will give him.
My current pony had similar arena issues to what you describe, but not nearly to the same extent. He had minimal brakes or steering, and was attached at the hip to the gate. I did lots of NH style groundwork (Clinton Anderson's Lunging for Respect stage 1 and 2 are great) near the gate, and trotted him away to rest at a different spot in the arena to rest and be praised. I repeated the process under saddle; work by the gate, rest somewhere else. The gate sourness is all but gone; the only remnant is a tendancy to cut deeper into the corner by the gate when he's ready to be done.
I also did lots of "sloppy rein" work; just holding the reins by the buckle, put my hand low, and let him trot. I grab the pommel of the saddle if I feel off balance, one rein stop if he goes too fast. Usually, if Scout is full of himself, 5 minutes of that will fix him to the point of a relatively responsive frame of mind, ready to work and respond willingly. For you, however, I recommend doing this in a VERY enclosed area (i.e. Roundpen.). You don't want to end up in a runaway situation, a very real possibility here. The point is, there is nothing for the horse to brace on and pull against, no "holding" back, an allowing to go forward, but an insistance to remain in gait (via one-rein-stop). It takes a VERY secure seat, though, to ride out a fast, strung-out trot, and potential sudden turns. Please, PLEASE be careful with this, stay in the arena until you have good control, and get help if it gets any worse at all.
A stubborn horse walks behind you, an impatient one in front of you, but a noble companion walks beside you ~ Unknown