You've probably seen these before, but it bears repeating.
Below are three (of many) videos that remind us the 'ideal' of feminine beauty is an impossible standard. As much as we can talk about it being ridiculous, though, it'd hard to internalize any alternative, as we are conditioned from a young age to how the marketing industry wants us to look (and buy, and act).
It's hard to fight the daily onslaught of imagery about how we are 'supposed' to look, but the first step (as always) is about being aware of its existence.
Women in media/advertising (and how that sets an impossible standard- literally):
Dove's fantastic campaign to open the doors to how the beauty industry manipulates reality:
Dove's second video, a bit more visceral as to what images girls are exposed to in conditioning what the ideal woman should be:
The best way to raise your self-esteem is to focus on your achievements, intellectually and physically, as opposed to your image.
Do things that make you feel good (like volunteering, making other people smile, being productive, etc). Move your body, regularly, so you are filled with oxygen and energy and you will feel empowered (even if you're not getting skinny).
Make things possible, and make them probable, and get them done.
And whenever you look in the mirror and that voice tells you you're so fat, ugly, wrinkled, plain, etc, think about those amazing things you can and have done. Because what you can do is what makes you powerful, and beautiful.