I used to be like that. Honestly, the best medicine for that is practice, practice, practice, and then fake it until you make it.
Your first interviews are going to be awful if you’re shy and awkward. You don’t know what to say, you don’t know how and when to say it, how to answer the questions correctly, etc.
My tip is to do a debriefing by yourself after each interview you do. Think about what happened, not in a omg-I-did-that way, but in a next-time-I-should-do/say-that way. For example, they asked you why you wanted to work there, or whatever, and you didn’t expect that. So you answered awkwardly and it screwed up the interview and you didn’t get the job because of that.
Then you go home, and you think about how you could have answered that correctly, and write it down. Also write down whatever you could have done better, or better answers to common questions they ask (they always ask the same things after a while). Then read your notes, and visualize yourself answering those questions with what you wrote down. Next interview, they’ll ask the same question or something similar, and you’ll have your answer prepared and have the confidence that your answer is good because you’ve spent hours thinking about it, and you visualized yourself saying it.
Also, on the phone: make notes of the points you want to talk about, your questions, the info you want to say to the other person, so that when you speak or leave a message, you don’t have to think about if you forgot something, it’s all right there in front of you. You forgot your phone number? Next time, have it written on a paper in front of you while you make the call.
So basically, practice by writing down answers and by reading them before the interviews. Just don’t bring your notes to the interview, of course.
About the “talking to people in person terrifies me”. How often do you talk to a new person? This is also about practice. Try talking to a person you don’t know and will never see again, just to practice. Then try it again with someone else. Then with someone else, and so on. Practice and seeing that it won’t always go wrong will make you gain a lot of confidence and see that in the end, it’s not a big deal.
Never say stuff like that. This affects your confidence a lot, and the way people see you. You’re 21, you probably know more than a lot of people.