This is an issue a lot of people come across at some point in their lives, and that is what they think about themselves. Some people are uncomfortable with the way they look, whether it be if they have imperfections they do not like, or if they are sensitive about their weight. They may even feel insecure about the way they do things differently to other people. All of these problems come down to one thing, and that is self confidence, one thing some people find hard to develop.
I think the pressures we get as a teenager are way too harsh, but unfortunately it's not something that can be changed easily. Most people have this image of perfection in their head and that is why they feel they aren't good enough. The pressure of being 'Skinny' is one that particularly effects girls and can be vicious cause of Anorexia and Bulimia, and this is because some people don't understand that 'we are who we are'
Although it may not seem like it, there is also a great pressure on boys when it comes to being attractive, after-all make-up is often noted down as a thing for Females, and while most girls do not use it to totally change how they look, much rather just bring out their best features, boys simply don't have that luxury (without the endless harassment for wearing male makeup anyway)
My best advice is probably that, although you may feel shit about certain things, there comes a point where you just think "I really don't care anymore" and once you get to this point, life is honestly so much better. At the end of the day, how can you expect other people to accept you when you don't accept yourself.
