You got to get a hang of it!! I'm guessing your very young. You have a while life ahead of you and it's going to be involving tons of interviews. You have to buld of confidence!! You should already know that a Manager isn't going to hire someone who isn't confident at all....
Try your best even if you have to fake it!!!
Practice in front of your mirror or to one of your friends...then you will eventually get a hang of it.
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Miles away but closer then ever