Keep in mind that breast growth tends to correspond with other puberty changes. Once you get your period, your breasts will start to grow to their full size. Mine sure did, and now I long for the days when it didn’t hurt to exercise and I could get dressed without strapping into a bra. The grass is always greener, right?
Even if your breasts never get bigger than they are now, you’ll learn to see that it’s really no big deal. Having a flat chest is sort of like having jerk radar. If girls ever make fun of you, then you know to stay away from them. And your small breasts will automatically deflect those creepy guys who only see women in terms of their fat deposits. (Plenty of guys couldn't care less about breast size. It's harder to find them, though, because they actually have manners.)
Something that made me feel better when I was flat-chested was realizing how many gorgeous, flat-chested women are out there. Take a look at this slide show, for instance. It's a reminder that women can be sexy and beautiful in about 5,000 different ways - including their small breasts. All that's keeping you from feeling sexy and beautiful, too, is your attitude.
Note: if you have a question about your body, your confidence or anything else in the world, come ask the community at the Teen Advice forum. Just don't ask for help with your calculus homework. There are limits to our knowledge, you know.