What to say to an insecure girls?

Tell her she's really nice and pretty, and she should have more confidence because she's special

Tell her she's the most beautiful person you've ever seen and that you really like her and would like to get to know her more.

I consent with the comment above for it's really sad that some people are so insecure and worried about small things that they can't even smile. One of the girls that I knew before I moved had her emotions literally locked up and never smiled. But when I gave her some confidence and spent some time with her she opened up and was amazing at everything that she did and because of my actions she is now smiling and was one of the most beautiful girls that I ever known. Basically you should give them some confidence, spend time with them, when they get stressed out look at their work and find out that what they did was beautiful and was not a piece of crap as she used to say . (hoped that I helped but sorry if I did not)