OP--Could be a dental problem, could be a physical problem as others have suggested, could (very likely) be a training problem. I don't have a whole lot of advice on the bitting end except to get out of the Tom Thumb. Maybe start with a simple snaffle, just to see how your suppling and flexibility is going with the plainest bit of all. If you have a coach or trainer, enlist their help. I imagine this horse needs a solid course in the basics of giving the pressure, bending, etc. When you get going consistent again, you might look at a dogbone Jr. Cowhorse as a starting place for a curb bit.
Wait, one more thing to throw out--what about a Little S hackamore?
Originally Posted by tinyliny
Can you tell me how that bit is different from the Tom Thumb? I see fundamentally the same sort of bit, though the curves differ a bit and the twist is different, of course.
And the gag--there's one huge difference in action right off the bat. It'll pull through the mouth, gradually tightening on the lips and bars, adding poll and curb pressure as it goes and pulls back and up on the tongue. Then you have far less leverage in the Cervi than in the TT. Why? Although the overall cheeks are longer, the ratio of shank to purchase is 1:1 as opposed to, what, 4:1 in your average TT? The magnification of leverage pressure will be negligible on the Cervi (at its maximum when the gag is fully engaged, but still probably only a quarter or third again added) while the average TT is going to multiple pressure on the reins fourfold. That's a lot of pressure on the jaw. Along with the gag mentioned earlier, you've also got the obvious "loose jaw," which is always a huge advantage for curb bits, lending itself to ease and subtlety of cues. A better finished mouthpiece with a smaller, more mobile central joint seals the deal. I disagree with kevin that the Cervi bits are the stuff of nightmares. Save for the twisted and too-thin smooth mouthpieces, they are definitely the lighter, better, less harsh bits of the two discussed.