Don't use a curb bit for now, but switch to a jointed snaffle. Preferrably a Full Cheek or a D ring. Why? Because your horse has basically learned to run through your bit AND a curb does not allow for proper application of Lateral movement, which is what you'll need to go back to in order to change this habit.
Basically what's happened is this...you've trained your horse to be a race horse. The way jockeys ride race horses in races is they pick up both reins and hold the horse back (so to speak) and at the last stretch, they let the horse out, release some of the slack. The horse feels less pressure in his mouth so he associates less or no pressure with "must go faster!"
You've been riding with the brakes on for so long that pressure now means nothing = you feel you need to hold onto both reins & hold back even harder. He fights it, goes to the front anyway & ends up aggitated.
Less is definitely more in this case. The less pressure you use the more important it becomes when you use it. Right now, pressure means nothing to the horse, so lack of pressure means "go faster!"
Get a snaffle bit and as Koom said...SERPENTINES!
They don't have to be perfect but they do need to give the horse a moment of release if they're going to work. That means, if you are picking up both reins and just directing his nervous energy without ever offering a release of pressure, they won't work and he'll be just as aggitated as if you used a curb and held him back.
You pick up ONE rein only. Let's say the left rein. You run your left hand down the left rein use your right hand to pull more rein through your left hand (don't want to lean forward!).
pick up the one rein and get his head to come around (bend the neck) and his left hip to move slightly (idealy: do this in an open space first, in an arena, then when you can do serpentines proficiently, go to the trail)
you want a BEND in the body. and to feel like he's not pulling against you. if he pulls against you, you can't release. you must hold and wait til he gives to the bit and bends slightly his body around your (left) leg (for the left rein) and then release.
1. drive the horse forward with two legs (squeeze)
2. pick up one rein
3. feel no pull from him as his head comes around (and you see half his face at least) and his hip moves over (back feet cross)
Ideal: he bends around your leg with each time you pick up the rein (be sure to put a slight leg cue to picking up the rein. you don't want to pick up the rein ever...without putting at least a little leg on him always. this keeps him from getting heavy)
Do the above in an arena/open area. Then when you can pick up the rein and get the wanted response every single time, no matter which rein you pick up, then go on the trail.
Ideal trail ride: don't go head to tail. but go around trees, bushes, etc... put his energy to good use. it's when you try to quell that energy "stop stop stop or slow slow slow" that just puts pressure on him and he responds by fighting it. Use the energy he's giving you and he'll relax faster.
Ask him to stop, back up 5 steps, trot up, stop, move the hip over, go forward at a walk, side step once, stop, back, move the shoulder over, trot, back up, etc. etc.....
He's not at the point yet where you can simply throw the reins away and let him go on auto pilot. He needs a job. He's got energy to burn and nothing to do with it. So, put it to good use and practice your moves.
Soon, if you are consistent, then if you want...you can switch to the curb bit (frankly, you're safer in a snaffle because it offers a wider range of control with lateral movement which is what gives you real control).
A curb bit is for a truly finished horse and rider, who work off seat and legs and very very little rein pressure.
You're safer in a snaffle and your horse will relearn to respect bit pressure if you go with something that's less severe. So, if you choose later to ride in a curb (it's ideal to switch back and forth between a snaffle and a curb if you want to ride in a curb)....your horse will respond to it.
curb bit: if you use the bit how it's intended: opening and closing your fingers only. no pulling, no harder pressure, then for ever ounce of pressure, your horse feels several more times as much pressure so that's why you must be light-handed if you use a curb bit.
you must be proficient at using seat and legs and your horse should be very responsive to seat and leg and of course rein. Also, pressure points with curbs are: in the mouth (tongue, bars, pallet maybe), poll, jaw. Also, the longer the shanks the higher the severity (how many pounds of pressure the horse really feels)
snaffle bit: if you need to really put let's say 5 pounds of pressure on one rein then your horse only feels 5 pounds of pressure and using it one rein at a time means that your horse only feels pressure on one part of his face (full cheek or d ring snaffles, which is why they are my favorite)....the pressure is 1 place: against the outside of the mouth. that's it. So, it's easier for the horse to understand and accept. And if you really need to bring his head around to your boot toe, you can, without damaging his mouth or using too much pressure all over his head and none in his mouth. (compared to a ring snaffle or an egg butt. these work on the inside of the mouth. they're fine, I just prefer the fc or d)