Act confidently to think confidently. This then feeds itself and turns into a never-ending cycle. It’s like the old adage says… Fake it until you make it.
It’s amazing what your body can trick your mind into believing. You can know what you’re talking about, but if you act meek or shy, if you don’t keep good posture, a smile on your face, or walk tall, if you have a limp handshake and don’t look people in the eyes, you’re not inspiring confidence in yourself. People around you see it and you feel it. It’s tough to get out of that cycle. People will be less likely trust you with big decisions, and if you run into the wrong people, they’ll look to take advantage of your lack of confidence.
Meanwhile, if you act confidently, speaking with purpose (loud, but not too loud; crisp; relatively quickly, but not too fast; with energy and enthusiasm, but not so much as to annoy the other person listening), keep your head up, your shoulders pulled back, look people in the eyes, you dress the part, and you genuinely sound like an expert, you’ll see much more success. People will be drawn to you.
Bottom line: act how you want to feel. Your body will lead the way.