Should I tell an adult? Can't I just handle this on my own?
Talking about problems within your relationship or sharing the fact that someone is abusing you is never easy, especially when you're talking to an adult. It's normal to want to solve your problems on your own or with your friends. It's normal to not want to get anyone "in trouble," including the person abusing you. If you are the friend of someone being abused, it's normal not to want to betray your friend's trust by revealing the abuse to an adult.
Sometimes, however, there are problems that are too big to handle without help, and it can be a big relief to involve a trusted adult. The advocates at loveisrespect.org can help you figure out when you should talk to an adult, who you should talk to, and what to say.