I think your friend has it right. I have met several people who seemed confident--almost cocky--at first, but once I got to know them and they began to trust me, their own insecurities began surfacing in our conversations. Turns out they were "faking it til they made it", too. So while it worked for them in that they felt strong
most of the time, I don't think anyone is 100% confident, 100% of the time. Even Angelina and Brad.
It may help you to make a list of things you're proud of, or things you like about yourself. Make them into a sort of mantra, to repeat to yourself when you start feeling insecure. It's worked for me.
Hope this helped! *hug*