For me, when that "confidence" spigot turned on, my life changed forever.
Me too. I was the quiet guy that never spoke unless spoken to. Who wants to hear what I have to say, was my guide.
I used to care what people thought of me, and like Richard says, that will show up in your voice and your whole demeanor. That's especially true in any kind of sales presentation.
Then I realized that not only do people not care about you, they won't remember what you said to them for more than a few seconds. That was liberating for me, and meant that I could blurt out just about anything and nobody would care.
Most people are hungry for conversation, which means that you can approach anybody and ask them anything, and then stand there and listen to them prattle on as long as you can stand it. "Oh really?", "How did you handle that?" and keep shining the spotlight on them, and they'll think you're the greatest person they ever met. When it's over, you'll know all about how their last three cats died, and they'll know absolutely nothing about you except that you're the nicest guy they ever met and a wonderful conversationalist.
A really good salesperson doesn't sell. He allows the customer to buy when he's ready, from a person he likes and trusts.
My advice is to stop caring about the outcome of any encounter. Buy if you want, leave if you don't, I don't care. Meanwhile, I'll keep you talking about yourself.
Being confident in every situation is the most fun I've ever had.