It is absolutely crucial that you DO have confidence as a director. And it can be achieved. I'll tell you how I do it.
Think about your film. Think, think, think and think about your film. Think about your shot. Think about what you will cut to. Think about where the window is going to be relative to the actor, and you will know which side of the face will be lit. Think about what time of day it is. Maybe its night time and the window is dark. Point is, you just think your film to death from beginning to end, every second of it. You will know in your head, exactly what you want the final product to be.
Let's say I'm an expert and I come to your set to help you and then I tell you a lot of things about how to shoot your film. You should not listen to me (unless your idea is actually a mistake, as opposed to artistic difference). Your film is not about MY expert vision. It should be about YOUR vision, whatever it is that you were thinking about when you went to bed lasst night thinking about your film. Your vision doesn't have to be perfect, but it's YOUR vision. You keep doing it your non-expert way, and sooner or later, you'll get better, and you'll refine your vision.
You HAVE to know what you want. Deep inside, you'll just have to know it. When you think about your film enough, trust me, you'll know it. And when you know it, other people will feel it, and I'm telling you, they will all fall in line, and they will enjoy doing it your way, because they'll be confident that you know what you're doing. They'll stop arguing with you. But for that to happen, you HAVE to know what you want.
Do not ask for advice. It feels like a logical thing to do when you have more expereienced people around you. But don't do it. It will make people think that you don't know what you're doing. You don't want that to happen. Do all your asking before the shoot, not during the shoot. If you don't know enough experts, come here and ask. But do it BEFORE your actual shoot.
Edit: That joke by Directorik about the lightbulb is as serious as it gets. The words "What do you think?" should not escape your lips during the shoot. It's about what YOU think. You're allowed to change your mind. Everybody is allowed. If you decide something you said, is not going to work, change your mind. Tell everybody "I changed my mind." It's allowed. Do not ask other people what they think.
I have to go and can't write more right now, but a quick way to get started is to go work on other people's sets. See how they do it. If you've seen how other people do it, you'll have indirect experience. Enough to argue about an issue with an expert who is opposed to something you're doing. And also, when you're thinking about your film, it'll be based on experience, and it's more likely that your thinking will be correct. If something you planned isn't working, get rid of that idea. Even if it hurts. It's all in the name of a finished final product, in a limited amount of time.
People are different. Some people command the attention of a room when they walk in, some people have to work at it. It's just the way it is. But you can command your set. It is not important that you communicate every time, WHY you are doing something. Sometimes there is not enough time. Some days will be better than others, but if you really, really work hard on your film, on every aspect of it, even aspects that other people are responsible for, you'll have more than the requisite confidence. You'll know if somebody is making a mistake, or doing something that is incongruent with something else some other person is doing, and you'll be able to do the necessary course-correction before things get ruined.
Best of luck to you. You just have to know your sh*t (go to other people's sets, watch tutorials on youtube), and know what you want. You should not care about what other people want. It's a nice thing to care about what other people want in your every day life. It's not necessary on a film set, that has time constraints.
You should not take anything I said as good advice. It's just my way of doing things. I'm generally confident about my thoughts and ideas, but I have stage fright. I was extremely afraid that I would fail to control my shoots, with so many unknown people. So far I've been happy, with even my bad days, on how things went, by the end of the day.
A couple of people don't work with me anymore. And it's alright. It's not personal. Everybody has an ego. But if other people can have egos, so can you. Don't deprive yourself of your ego, because other people have theirs. Do not joke around on your shoot too much. Sooner or later you'll say something that offends somebody. One of the people I mentioned above was offended by a joke I believe, and he was right. Have fun, but don't make unnecessary jokes.
I really do have to go now. Best of luck to you. And again, don't take anything I said as expert advice. Evaluate for yourself. Also know, that most filmmakers disagree with me on most things, and I disagree with them just as much. And it's all okay. You just have to do it your way. Let them do it their way. But always be willing to accept somebody else's argument, and change your mind. Most people are there to help you. Just believe it.
Cheers

Aveek