Re: What is holding you back?
I think what's holding us back is that we may not have experienced a true Kiai. I had a judo sensei who described his situation of having been trained in a non-yelling tradition - no shouting, no growling, just training and working quietly on techniques.
Then, he said, once (in about 20 years of judo) he was in a shiai, and he wasn't sure what "clicked" but this odd sounding roar came out mid-throw, the technique was executed cleanly, fast, dynamically, and the point was earned with no doubt whatsoever. He said that the sound he produced came up from his diaphragm or below, and he'd never tried to do anything like it.
I think THAT may have been a "real" kiai, and most of the shouting we do in dojo all over the world is just that - shouting.
Consciously trying to yell something isn't necessary a kiai. producing power in a conscious/unconscious movement - may or may not be accompanied by a shout, but there's probably a kiai in there, whether you hear it or not...
But that's IMO..