I can't really give advice as I don't think I've had enough experience. But I'll just give you my point of view. I think all you need to have confidence is prepare. If you prepare hard enough and if you know 'exactly' what you want and are able to recognize when things aren't happening as you want them and you immediately correct it, then the people around you will know that you know what you're doing and fall in line. You don't really have to learn to be confident, it'll be automatic.
Even then you'll have some people who think you don't know what you're doing because they've worked in big productions with big directors, you just kind of have to not pick a fight, but to just make sure that they do what you want them to do. They will eventually listen to you as everybody else is also listening to you. Mob mentality will prevail on set, in a good way if you know what you're doing, in a bad way if you don't.