bbc.com
Among 10 to 15-year-old girls, the charity's report says 14% are unhappy with their lives as a whole, and 34% with their appearance.
Researchers were told of girls feeling ugly or worthless.
The figures for England, Wales and Scotland for 2013-14 represent a sharp rise in unhappiness on five years before.
By contrast the study found that boys' sense of happiness remained stable.
Megan, 12, said: "The only time that I'm not happy is if people are judging me or being mean and things like that. With people at school, they post things [on social media] and they try and make everyone think that they are perfect.
"Sometimes it makes me feel - not annoyed - but I don't want to look at it any more because they just do it all the time and it gets on your nerves."
Natalia, 15, said: "Everywhere you look it's like, celebrities: thin, blonde or - perfect teeth, perfect hair, perfect eyes, perfect eyebrows. And it's just crazy and I just feel like I should look like that - even though I know it's all like fake, or a lot of it is anyway.
"I have these days when I'm like, I don't care what people think but then somebody will say something and it will just hit me again and I'll feel worse but I don't know, it's hard to explain why it bothers me so much sometimes."
Caitlyn, 12, said: "I am happy most of the time, but then when it comes to my friends going: 'Ah I look really beautiful in this outfit' and everything, I just feel like, no, I can't do that - I can't pull it off.
"When I'm obviously looking through my
Facebook and looking at some of the posts, all you can see is pictures of celebrities and my friends looking beautiful in selfies and everything, and then there's just me, like, I can't get away from any of it."