Your confidence will depend on how strongly you believe in your intention. Your intention4 is your goal or desire. Without intent any action is useless and meaningless, and your words will be utterly devoid of power. To be successful you need to formulate, focus on, and put all your mental energy towards your intention.
Think of it this way; although we know we can never truly be certain as to the exact outcome of any event, we can be certain about our intent. Your intent will give you outcome you want to achieve. Just focus on that and nothing else.
Also it helps, before you do anything, to go inside and say to yourself confidently and powerfully, in the most authoritative voice, 'Of course I can do this!'
Say that again and again so that you believe it, 'Of course I can do this". You'll notice that the mere fact that you are saying that inside your mind that causes you to accept and believe the fact that you will be successful. So, while we can never truly be certain about the specifics of any interaction or outcome, we can always be certain about our intent.
I also want to point out this important fact - confidence isn't certainty. This is true for anything in life. You can be confident, but you can never be certain of anything.
I know that I'm very confident, but even with all my confidence, sometimes things don't work out the way I want them to. That happens. However I also know that even though I am confident I know HOW to do something, and I have the ABILITY to do it, if it doesn't work, it is not going to erode my confidence. I know if I am consistent and focus on my intent the things WILL happen.
My point here is this - never get discouraged. With a little practice and focusing on your intent, you WILL be very successful. So just "BE" confident and the rest will follow.