Again I am going to say it, try a bitless bridle. If you are too insecure on your horse then no bit will work. I have never seen a horse that is not happier in a bitless bridle and if you use your cues properly you DO NOT NEED A BIT. I think if you could just try one you would find your horse to be much, much happier, relaxed and very willing. You do not need a bit to get the correct headset, or for collection or to do reigning.
Actually, I've had horses go both ways. Loki hates
bitless. He just doesn't like working in it, he's much happier with something in his mouth. I'm actually thinking about getting a double jointed, or a straight bar, like Clouds mentioned. He's a bit of a quirk, he'd probably be pleased as pie with a solid mouthpiece. He does very well in my regular eggbutt, but why not make him even happier?
On the other side, Sammy (TWH) hated
bits (teeth fine)! He did relatively well in a snaffle, but was much happier in just a simple halter, or fluffy nosepieced mechanical hackamore. He neck reined and worked very well off the legs, so it worked fine with him. If he was greener in terms of working off cues, I would have looked for a more appropriate bitless setup.
But, since he was essentially finished, he didn't require anything special. I felt most comfortable working in a halter with him, it allowed me to lunge him easily and was fine if I needed to use the reins to whoa. Light pressure, much less than if I was asking him to back on the ground, was enough to work him.
It really varies from horse to horse. My QH mare works fine either way, she's pleased in her snaffle, or just working around in a halter. Again, she's more finished in cues, so she doesn't require a special or really proper setup.