Everyone knows that dancers (especially ballerinas) strive for an ideal look: tall(ish), lean, toned (but not obscenely muscular), strong, and stable. Yet among all of this, you're expected to be delicate and graceful or angular and hard-hitting, depending on what you're being asked to do. Impossible? Probably.
This ideal is strangely unhealthy for dancers. I did a post about a month and a half back about the ideal feet, but I realize that this isn't the major problem. What the real problem is is image and learning to accept and work with what you've got.
There's a reason why there's a huge culture of nutritional issues and structural issues in dance: no other profession forces you to stare into a mirror as long as dance does. You're spending so much time looking at yourself, criticizing yourself, analyzing your every imperfection - you think that's not going to affect you? You've done it probably from a young age, and it's encouraged for you to look in that mirror and see the ugliness, not the beauty, in yourself.
Sure, not every dancer has an eating disorder or is working out beyond his/her limits in order to attain the perfect body, but every dancer has something(s) they would like to change. Whether it be the arch of your back, or your shoulder alignment, or the width of your waist, you can probably think of something right now that you want to change. Take a moment. Write it down if you like. One thing, the biggest thing you want to change.
Got it? Now, go to a mirror. Stand in front of it and note the things about your body that are good for dancing and/or aesthetically pleasing to you. Do you have high arches? Maybe you have an hourglass figure, or maybe you were born with the "ballet body" - lean and tall. Have you got strong, square shoulders or hips? There's something pleasing about every body.
Now just think, every time you stand in the studio in front of the mirror criticizing yourself, what if you took a moment to praise yourself as well? Or at least turn that "oh my god, my butt looks huge" criticism into a "maybe I'm arching my back too much and sticking my butt out" constructive criticism. If you're not pleased with what you see in the mirror, try changing your alignment. Odds are that if you're perfectly stacked head over shoulders over pelvis over ball of foot (or feet), you will look perfectly fine.
I could go on and on about positive thinking, but it won't do any good unless you also accept the fact that you have faults. There is no perfect body - the gumby ballerina you're jealous of most likely had to spend years building up her strength so that she could support her stretchy limbs. That girl with high arches in your class will have a much harder time balancing than you, with lower arches. Or if you're the gumby one, or the girl with high arches, you were naturally born with the aesthetic that dance seems to demand (and believe me, I'm jealous). There's a con to every pro, but also remember that to every pro, there is a con. Accept both your pros and your cons and use your pros to strengthen your cons. See the beauty, through everything. Once you change your outlook from criticizing to constructive, the mirror won't be so scary after all.