- Practice makes perfect: find someone to give you a mock interview. Do this as often as necessary until you conquer your problem. - Caffeine, adderall, etc, can make you much more anxious. You need to be calm. - Don't feel pressure/rushed, and don't panic. Slow down. If your mind races under pressure, that's completely normal, but it can be controlled. - Its ok to not entirely know the answer. Start by responding with what you do know. Once you begin speaking, the ideas will start flowing. Many technical questions are "loaded". To actually solve the problem in the best way possible, you need way more information than they give you. This will cause your mind to race. Try to qualify your response with your thought process, "if this or this, I'd do this. If something or another, I'd probably do this." - Practice speaking with confidence. Take a deep breath, all the way down into your belly, and then use that air pressure to make your vocal chords "sing". Having a strong vocal tonality will resonate confidence inside your head, and inside theirs. Also, don't be monotone/boring. Find/express energy and enthusiasm without sounding fake or cheesy.
Being nervous for an interview is normal. I've interviewed a lot, and I always get nervous. Its just something you have to deal with. Public speakers and pick up artists (approaching women to ask them out) are two areas that you can find a ton of advice for calming your nerves.
Introverts can have a hard time finding their inner personality. Practice makes perfect. Try to start as many conversations as you can each day. Don't force it, become natural at engaging conversations with other people. This will help all areas of your life, including interviews.