Hey there, as a guy in my early/mid 20's who hasnt had very much luck with girls I would say to always be yourself. Don't EVER try to be someone your not just to try and impress some guy. Thats not the type of guy you want to be near. You want to be with somebody who likes you for who YOU are.
Yes obviously looks are important, and it is what immediately attracts a guy to a girl. But thats just the beginning and you will be amazed with the amount of people i at first didnt find attractive and then once I got to know them, I became so much more attracted to them, not just personality wise, but also looks wise. Dont get hung up on your looks. I'm sure there are plenty of people who find you attractive.
What I value in a girl: politeness, a girl who is lady-like, respectful, honest, trustworthy, loyal, interesting to talk to (i.e. has more interests outside of shoes, clothes, and america's next top model lol), funny (in their own way, not some clown who wants to be the centre of attention trying to impress everyone).
Never think about your breast size etc etc again. Any guy worth having wont care about stuff like that. Obviously if you feel you are too thin and you aren't happy about that then you can change that, BUT do it for YOU, and for nobody else.
By the sounds of thing you are a very nice girl so dont get hung up over shallow guys who dont look beyond people's looks. You'll find somebody great who appreciates you. Keep the chin up.
I just wish I could take my own advice