I agree with this. In all areas of MA, real confidence tends to come from the experience of losing and surviving. From knowing you're actually challenging your skills, putting them to the test, and re-proving them as you do so. To do that, you have to expose yourself to the chance to lose (otherwise, you end up never really putting them to the test). And if you expose yourself to that chance enough, you're going to lose some.It comes back to being prepared to loose. In training especially, people are scared to take risks for fear of the consequences. The confidence comes in making that leap fearlessly.
Grappling is very common for this because to advance position you often have to risk loosing position.
Loosing position means either looking like a fool or being underneath a fat sweaty guy. Or both.