A girl's confidence starts with you.
"I plead with you young women to please be more accepting of yourselves, including your body shape and style, with a little less longing to look like someone else. . . . Every young woman is a child of destiny and every adult woman a powerful force for good. I mention adult women because, sisters, you are our greatest examples and resource for these young women. And if you are obsessing over being a size 2, you won’t be very surprised when your daughter or the Mia Maid in your class does the same and makes herself physically ill trying to accomplish it."
-Elder Jeffrey R. Holland
When you say something negative about yourself, the instant effect is that you make everyone else around you feel self-conscious. Your comment might cause them to fixate on a similar thing they don't like about themselves or to feel awkward because they don't know how to respond or to start re-examining themselves because they didn't realize they should be self-conscious about something like that.
See how it spins into a vicious cycle of self-loathing? Nothing positive comes from speaking negatively about yourself or others, and other people learn from your behaviors.