Hi. I understand what you are going through. Although I've always been proud of my breast size (34DD), growing up it does cause a lot of sexuality issues. While most of your friends talk non-stop about ways of making themselves more attractive for the boys in your school, you probably notice you are getting a lot of attention from older men. That made me gain weight, to make myself less attractive (which didn't work, since it only made my boobs bigger).
I know you think all other girls are ok with their bodies, but trust me, as you grow up and get more self confidence, you will realize noone is 100% comfortable with who they are.
When you go shopping, enjoy it as much as anyone else, because you deserve everything all the other girls have. Find clothes that you feel pretty and comfortable in, but don't hide yourself.
The only thibng that may be of concern is your back pain. For that, I think you should see a doctor.
All that said, all teenagers have low self steem or insecurities

it comes with adolescence. You never fully recover, haha, but it does get much much better with the years.
And I am sure you are super smart. Don't try to make yourself ugly so people will take yourself seriously. You take yourself seriously and people will do the same.
I am sorry for this long post, hopefully I helped a bit.
Have fun