wow, that sucks. his brother is an ass and yes you should definitely stay away from him and any guys like him. sex is not a substitute for love or self-esteem. you need to feel good about yourself and you obviously don't have good self-esteem if you're letting guys treat you this way.
tell your best friend how sorry you are and how much you regret it and explain to him why it happened. tell him how it makes you feel when you have sex - that it helps you feel better about your body but that you realize it's not the solution. he'll forgive you eventually but you need to keep at it and not give up.
he's selfish that's why. decide that you deserve better and get rid of him.
i've felt like this nearly every day of my life and i'm probably a lot older than you. i don't know if i have any advice for you. i can only say that what you probably already know - your self esteem is incredibly low. if you can afford some therapy - get some. if not, try to focus on other things in your life that you're good at or things you would like to be good at. try to take the focus away from your looks and more to who you are and who you want to become...is there anything you want to accomplish? maybe in sports? music? writing? dance? focus on something you want to excel at..something that you would enjoy.
looks don't last forever even for those who have them so try to work on your person instead. that being said, it's much easier said than done - trust me, i know! just try to keep at it.
join the club! it isn't anyone's fault. unfortunately, vaginal sex just doesn't hit 'the spot' for most women. try some manual stimulation while he's inside of you or get him to do that.
you could also buy a cock ring that has a small vibrator on top so that while he's in you the vibrator is on your clit. good luck!